Run the game with '--noWorkshop' (without the quote marks) as the launch parameter under Steam > library > Rightclick on Cities Skylines > Properties > Set Launch Options.

If the game works fine, check your mods.

• Windows updates have a habit of corrupting the videocard driver. If your game stops working after a Windows update, reinstall your videocard driver.
  
  
• Use this list to check if you have any broken/dead/incompatible mods. (copy the URL if the link doesn't work, because steam's link filter is not the smartest biscuit in the tin, and considers all google docs links evil. 🙄)
  https://docs.google.com/spreadsheets/d/1mVFkj_7ij4FLzKs2QJaONNmb9Z-SRqUeG6xFGqEX1ew/htmlview
  
  Compatibility Report is also a very helpful mod: https://sp.zhabite.com/sharedfiles/filedetails/?id=2881031511
  
  Every time the game is updated, most mods break. They need to be updated, by people who have lives, families and jobs. Wait at least 2 weeks before complaining about the fact your game doesn't work.
  
  
• NEVER DO A SECOND LOAD!
  Always quit to the desktop between loads! This is a Unity game. Which means you can’t do a second load if you mod the game.
  
  A second-load is loading another map/save when one has been loaded before in the current session, including after quitting to the main menu. The game loads mods in two halves, one part on the Paradox logo, and one part during the loading screen. By loading a second time, the game forces mods to load themselves twice, which causes problems.
  
  
• NEVER OVERWRITE A SAVE FILE.
  ALWAYS make a new save when you save your game.
  "Save often and in different slots." The oldest mantra in gaming history. This should be basic common sense, but apparently it's not.
  Make them incremental, so city_1, city_2, city_3, etc. Don't rely on one or two saves, they can become corrupted very easily, and you don't want to lose your entire city just because you only have one save.
  
  
  ALWAYS READ A MOD'S DESCRIPTION BEFORE SUBSCRIBING!

The main issues at the moment:

The new DLC's don't show up in my game.
Make sure you're using the most recent version of the Loading Screen Mod. the mod has been updated and now loads the new content.

Steam won't download workshop content. / "Missing Downloaded Files" error.
In the Steam Client, check the Downloads page. Is the download stuck there? Perhaps with a "Missing Downloaded Files" error?

If so, that happens on occasion, it's a corrupted temp download file. If you've got the Missing Downloaded Files error, try this. Usually wteps 1-4 are sufficient:

1. Close down steam completely (so Steam > Exit in the upper left menu),
2. go to C:\Program Files (x86)\Steam\steamapps\workshop.
3. Delete the Downloads folder.
4. Then restart Steam. Resume the download if needed.

If that didn't help:
5. Hover over the "Missing Download Files" text and note the Steam ID. (the string of numbers).
6. Repeat step 1 and 2.
7. Open the 255710 folder.
8. Delete the folder with the mod's SteamID.
9. Repeat step 3 and 4.

This will not affect your game, saves or workshop items. All you'll be deleting is a temporary file that's stored during the download. Normally this file is moved to the Workshop folder once completed, but because it's corrupt Steam doesn't recognize it and it stays in the Downloads folder.

Game crashes before the main menu
If you have Citrix Receiver or Citrix Workspace installed, uninstall it. The receiver installs its own virtual videocard adapter, which screws up Unity games.This is NOT a problem with the game, and NOT something for Paradox or colossal Order to fix. The only option is to uninstall it and reinstall it without the virtual GPU component. The Citrix devs are aware of it, but since it's mostly a business program (and notorious for being rubbish), I doubt we'll see a fix before
2020 2021 2022. (Citrix Workspace is the new name for Citrix Receiver.)

RivaTuner Statistics Server is also known to cause this.

Error Logs
The game creates an error log and an output log on crashes, and keeps an output log for everything it does. If you get a crash or an error in the game, always check the output log.

For a full crash:
Logs location after a crash:
C:\Program Files (x86)\Steam\steamapps\common\Cities_Skylines\31. dec. 1969 kl. 16:00[/i]

For an error:
When you get the error, DO NOT QUIT THE GAME. The second the error appears, leave it be, immediately alt+tab out of the game and locate the output log. This is an active log of everything that happens in the game, and if you do anything else, the error will be impossible to find. Make a copy of the file to your desktop or some other folder.

General output_log.txt is located in:
C:\Program Files (x86)\Steam\steamapps\common\Cities_Skylines\Cities_Data

Apple:
Users/<username>/Library/Logs/Unity/Player.log

Linux:
~/.config/unity3d/Colossal Order/Cities: Skylines/Player.log

Game crashes on Exit
If you use mods, the game will probably crash when you use the 'Exit Game' option. This is completely normal, and nothing to worry about.

Steam or The Workshop won't load
Always check https://steamstat.us/ first to see the status of the Steam Servers.

All my custom content is missing/disabled!
Yea, that happens.

When you quit the game, Steam updates itself, or if you subscribe/unsubscribe to a lot of workshop content, the file that contains all the information on which mods are enabled or disabled can become corrupted. The game wipes the file and leaves it as 0kb, at which point Windows automatically deletes it as a broken file. There's nothing you can do to prevent that. from happening.

This file is userGameState.cgs, located in %localappdata%\Colossal Order\Cities_Skylines.

If it happens a lot, you can make a backup of the file (so long as it's more than 0KB it's intact). If the game boots without everything enabled, you quit the game, copy the backup back into the folder, and re-launch the game.

If you've added any new workshop items since you made the backup, you will have to manually re-enable those. Note that this may corrupt the file again.

Mod related errors after load, missing concrete, etc
If you use mods, don't perform a second-load. Always quit to the desktop between loads.

A second-load is loading another map/save when one has been loaded before in the current session, including after quitting to the main menu. The game loads mods in two halves, one part on the Paradox logo, and one part during the loading screen. By loading a second time, the game forces mods to load themselves twice, which causes problems.

Error: Game is not available on your current platform
Cities Skylines can only run on x64(64bit) systems. x86(32bit) systems can not run the game! The only way to change a 32bit to a 64bit OS is to reinstall Windows with the 64bit version of Windows.

Error: Cities.exe has stopped working
Check the error logs in the folder with the date and time of the crash. There’s a good chance you ran out of RAM. All assets, both custom and vanilla, are loaded into the RAM on game load, and remain there the entire session. You will need to reduce the number of subscribed custom assets, or buy more RAM.

If your paging file is set to 0/off, turn it back ON. This game needs the paging file to run properly!

Lowering the Texture Quality setting also reduces RAM usage. If neither of the above options are possible, try lowering the Texture Quality before loading a city. (This downsamples the texture files, and thus uses less RAM. Don't change it back inside the game, it'll just freeze as it reloads all of the higher resolution ones into the RAM.)

Error: Object reference not set to an instance of an object [System.NullReferenceException]
Immediately alt+tab to the desktop and check the Output_log.txt in Cities_Data. Scroll all the way to the end and check the matching error.

Warning: You can no longer install RICO Leisure/Tourism buildings if you do not have the After Dark DLC!!!!!! Doing so will break your game!

Error: Array index is out of range [System.IndexOutOfRangeException]
Immediately alt+tab to the desktop and check the Output_log.txt in Cities_Data. Scroll all the way to the end and check the matching error.

Error: Broken Assets - LOD has too many vertices
The 1.3.0 update added a limit to the LOD. Old assets that go over that limit do not work. The original creator needs to fix this, there is nothing you can do. Unsubscribe the asset, it won’t work until it’s fixed.
To locate the asset: The error tells you exactly which asset is too highly detailed and won't work. The name of the asset is literally in the error. That string of numbers is the SteamID.

Just copy the error, paste it in Notepad, then copy the string of numbers. Open the workshop (in your browser), open any workshop item, and replace the string of numbers at the end with the one from the error. That'll take you straight to it. Unsub and problem solved.

The game won't launch at all
Add Cities.exe to the exceptions in your Antivirus program.
(Also: "Game disappears after I click Play")

Game crashes before the main menu
If you have Citrix Receiver or Citrix Workspace installed, uninstall it. The receiver installs its own virtual videocard adapter, which screws up Unity games. The only option is to uninstall it. The Citrix devs are aware of it, but since it's mostly a business program (and notorious for being rubbish), I doubt we'll see a fix before 2020. (Citrix Workspace is the new name for Citrix Receiver.)

RivaTuner Statistics Server is also known to cause this. Disable it.

Can’t get past the loading Screen
Please check your mods against this list of incompatible mods[docs.google.com].

If this is the second time you're loading something inside the game, quit to the desktop and restart the game. See 'Mod related errors after load' under the 'Errors' section.

Black screen on loading
If your game stops working after a Windows update, reinstall your videocard driver. Windows updates have a habit of corrupting the videocard driver.

The game crashes after it finished loading the game
Check the logs. The most common cause is not having enough RAM. You’ll either need more RAM, or less assets.

Steam doesn’t work
Always check https://steamstat.us/ if Steam is having issues.
The Steam Community Servers (which house the Workshop) undergo scheduled maintenance every Tuesday-to-Wednesday night around 00:00 (GMT, London time).
If there's a Steam Sale ongoing, the servers are notoriously unstable. You can't fix this, all you can do is wait.

The game stops loading halfway
Usually happens when you have two versions of Network Extensions installed, or Traffic++V2. Uninstall Traffic++V2 and the old version of Network Extensions. Disabling is not enough, parts of their code still run.

If this is the second time you're loading something inside the game, quit to the desktop and restart the game. See 'Mod related errors after load' under the 'Errors' section.

Game still won't load properly after all the above.
Try reverifying the game files. Open the Steam Client > Library > Rightclick on Cities Skylines > Properties > Local Files > click on Verify integrity of game files.

Loading takes hours
Use the Loading Screen Mod. The game normally only uses one CPU core to load everything. Loading Screen Mod enables multicore loading.

The game loads every mesh and texture for each building, including identical ones. So if 20 assets use the same texture and mesh of 10MB, it loads 200MB. The Loading Screen Mod allows buildings to share one instance of those, so it only loads 10MB once then links it to each file.

Why can't I get 60fps? [Game from a completely unrelated genre] runs perfectly!
Forget 60fps. It's not going to happen.

This is a static antfarm game. Not a shooter. You don't need more than 25fps.

This game is a simulator. That means it has to calculate and render every unit and item in this game at all times. Any other type of game it only bothers to render and calculate the current level you're playing, and the enemies on that specific level. And if you add mods and custom content? The fps will drop further the more you add.

In other genres you get roughly 10-30 bots in the active area at once.
This game? 16.000 vehicles, 65.000 citizen, 1.000.000 trees, 49.000 buildings....

Even Pizz Daint and TaihuLight would struggle with the game, because it's a simulator and it runs on Unity. Yes, Unity is an issue, but the real problem is the genre, not just the engine.

A game like this has to render and calculate the entire game at once, all the time. There is no commercial hardware available that can handle that.

60fps isn't going to happen. You'll be lucky to get 25fps. Get over it.

Strange graphical artifacts across the screen or extremely low FPS
Your system likely has an IntelHD or Radeon R5. This is an integrated GPU, and those are too weak.
Give the dx9 launch parameter a try. It might lower it down enough to be able to run. Apple users will need to use OpenGL instead, as new OSX versions don’t play nice with DirectX at all.

For Windows: Add '-force-d3d9' (without the quote marks) under Steam > library >rightclick on Cities Skylines > Properties > Set Launch Options.

For Apple: Add '-force-glcore' (without the quote marks) under Steam > library >rightclick on Cities Skylines > Properties > Set Launch Options.

Blurry Screen
They changed the Tilt Shift option to Depth Of Field in the 1.2.0 update, and added a few modes to it. By default it's set way up on the new mode. This turns the entire screen into one massive blur.

Set the Depth of Field option to Legacy, and turn it all the way down.

If you have any camera mods, those also cause issues with the Depth Of Field settings. Turning the game's DoF setting all the way down fixes the problem there as well.

Blue screen without city <Repeats every Game Update>
Every update generally breaks the Dynamic Resolution mod. Please disable it until it is fixed. Check the mod's description for updates.

No sound
This happens on some systems. There is no definitive answer, but here are a few things to try:

• Disable all audio managers. This includes ASUS Sonic Studio, Alienware Sound and other such programs. (Realtek Audio is fine, that's your main driver. Leave that enabled.)
  
• Disable Steam Streaming Speakers under your system's Sound Settings.
  
• Reinstall your Audio Driver. Especially after a Windows Update.
  
• Check your sound settings. Rightclick on the audio icon in the taskbar > Open Volume Mixer > Scroll to Cities Skylines and check the slider.
  
• Change the Audio Depth to 16 bit. Go to your system's or driver's audio settings > Advanced > change the depth to 16 bit, 44.1kHz. Save, then reboot your computer.

Can't speed up. Speed 2x and 3x don't work.
The game has reached the limits of your CPU. When this happens depends entirely on how powerful your CPU is. If you use heavy mods, or run something in the background, you will reach this limit sooner. Even the most powerful CPU has this limit. Nothing can be done about it.

FPS drops when opening beautification tab
Remove the More Beautification mod.
The 1.10 patch changed the way thumbnails are generated. Every time you open one of the tabs in the beautification panel, the game generates all the thumbnails. Use Find It! 2 instead.

Population drop
This game suffers from something we call Death Waves. You need to zone residential areas in small sections at a time to avoid them.

The problem is that Cims all move in at the same age, and die at the same age. There is no dynamic lifespan in the base game. If you zone an entire neighbourhood of 1000 people at once, those 1000 people will also die at once. New residents also won't move in until the dead people have been removed from the house.

Freeze on save
Do NOT click the mouse when the game hangs. Often it hasn't actually crashed, Windows just thinks it has. Savegames in CSL are pretty big, and it has to write several dozen MB's to an already maxed out system. Using the Steam Cloud can make things worse.

Floating trees
The Tree Snapping mod is not compatible with the Unlimited Trees mod. Using the two together causes your forests to fly. Remove one of the two.

All buildings are marked as Historical
Plop The Growables was updated, and now has the following option set as default:

• Make All Historical This option is ON by default. It makes all the growables in the game historical. This means they can level up but will not change appearance.

To restore the old system, turn the option off, then quit and relaunch your game. Read the mod's description for more information.

No Depots Found
Save and restart the game. It's a bug that we got in a recent version of the game. By restarting & reloading, the game rechecks the buildings and the depots you've placed will be recognized.

Snow icons on roads in non-winter maps
This is caused by loading custom snowplough vehicles into non-winter biomes. Unsibscribe them before loading a non-winter map/save.

How do zones & Demand work
(Also: "Demand is low")
There are 4 types of demand in the base game:
Residential, Commercial, Industrial Production and Office. The last two are both represented by the yellow Industrial bar.

Residential demand is generated by is people wanting to move in. This comes first.
Once people move in, they need groceries. This raises the Commercial demand.
Once shops grow, they need supplies. This is where Industrial Production comes in, producing Goods.
Production needs workers. This brings in new people, and causes Residential Demand to rise.
Once your city becomes more educated, people are going to start demanding white collar jobs. This causes the Office demand to rise.

There isn't a solid calculation for how much demand each generates, due to the varying size of houses and number of workspaces per building.

If nobody wants to move in, make sure there are enough jobs available for them.

Disable all mods you don't need before going in the editor. Most mods are not meant for the editors and can cause all sorts of problems.

Missing tabs in Editor
This is caused by the Ploppable RICO mod. This mod is not meant for the Editor, and should be disabled before opening one of the editors.
(Also: "UI missing in Editor")

Low framerate in Editor
Disable Ultimate Eyecandy before opening one of the editors.
(Also: Editor is slow)

Part of the screen unresponsive in Editor
Turn off Network Skins before opening one of the editors.

How do I use custom content?
Please follow the guide: Cities Skylines | A Beginner's Guide To Using Mods & Custom Content

Recommended mods for new players
The abovementioned Beginner's Guide has a comprehensive list of recommended mods. There is also
Avanya’s “Must Have” Guide, and my collection of Must-Have Mods for Beginners

Loading Screen Mod: Missing content?
The orange ones that appear on the left are missing props and trees. Missing props do not cause problems. They just don't appear on the actual building. So if you don't subscribe to a logo, the building will still work, just without the logo.

When they are missing, they do not use system resources; if you subscribe to them, they will use RAM. The ones in red as failed are a problem you need to fix.

Loading Screen Mod in the Editor
You can enable the Loading Screen Mod in the Editors by holding the left CTRL key pressed when you click the final button to launch the editor.

Loading Screen Mod Logging
You can enable the log for the Loading Screen Mod in its settings. It'll create a .htm file every time you load. When you open that file in a browser, you can check what the problem is and go directly to the file.

Log location is listed in the options. It's C:\Users\[YourUsername]\AppData\Local\Colossal Order\Cities_Skylines\Report\LoadingScreenMod

Locating and removing unused assets
There is a fairly easy way to see which assets aren't in use. All you need is the Loading Screen Mod.

1. Enable the log in the Loading Screen Mod.
   
2. Load your most recent save.
   
3. Once the city has loaded, quit the game.
   
4. Go to the folder with the Loading Screen Mod logs. (usually C:\Users\[Your Username]\AppData\Local\Colossal Order\Cities_Skylines\Reports)
   
5. Locate the log file. If there are more because the options was already enabled, select the most recent one. (sort by date is easiest)
   
6. Open the file in your browser.
   
7. Scroll down until you get to "The following loaded assets are currently unnecessary (not used in this city)" These are all the items you haven't used.
   
8. Click the name of any asset to be taken directly to the workshop page, where you can unsubscribe.

Note that props, trees and decals placed by hand are NOT counted as in use. So be careful with those, as it only sees the ones included in buildings as 'in use'. Buildings/Parks, Props/Decals, Trees, and Vehicles, are all listed seperately.

Can you recommend some good mods to get?
Avanya's collections of Must Have Mods are a good place to start.
https://sp.zhabite.com/sharedfiles/filedetails/?id=823267366

81 Tiles mod performance
There is no significant performance difference between 9, 25 and 81 tiles.

The entire 81 tile map is already loaded, calculated and rendered at all times without the mods, even when you're not looking at it and only have the starting tile unlocked. Unlocking them makes no difference other than removing the area boundary.

The mod will slow down your system only while unlocking the tiles. Which is normal, because it's running a script.

Any performance change after the unlocking comes from the player constructing more assets adding more citizens and creating more traffic than before it was unlocked.

RICO Local
Locally created RICO settings are stored in the game's main directory. (where cities.exe is).

Rico settings for buildings that came with RICO settings have a seperate .xml file inside their asset folder under steamapps\workshop\255710. You do not need to create local settings for them inside the game!!!!!

Prop It Up reset
All settings are saved in C:\Program Files (x86)\Steam\steamapps\common\Cities_Skylines\CSL_PropItUp.xml. You can either open it in any text editor and delete the specific entry or you can simply delete the entire .xml file to reset all Prop It Up settings. The game then creates the file anew.

Building Themes reset
All settings are saved in C:\Program Files (x86)\Steam\steamapps\common\Cities_Skylines\BuildingThemes.xml. You can either open it in any text editor and delete the specific entry or you can simply delete the entire .xml file to reset all Prop It Up settings. The game then creates the file anew.

Network Skins and Surface Painter have reset themselves
This happens if you subscribe & unsubscribe workshop items whilst the game is running. Quit the game completely before subscribing/unsubscribing workshop content.

Surfaces from the Ploppable Asphalt+ mod that have been converted to Procedural Objects sometimes disappear on load.
Cause: Incorrect load order. Procedural Objects mod attempts to create PO items made from PA+ surfaces that haven't been loaded yet.
Possible fixes:
- Move PA+ to your local mods folder so it loads first (all local mods load first before workshop mods)
- Unsubscribe and resubscribe to PO so it loads last (the most recently subbed/updated mods load last)
- Use the Load Order Mod alpha and make sure PO loads last
(Thanks Pewex for identifying the fix and kjmci for the guide entry!)

I've started keeping a list of broken and incompatible mods in Google Drive.
Use this list to check if you have any broken/dead/incompatible mods. (copy the URL, because steam's link filter is not the smartest biscuit in the tin, and considers all google docs links evil. 🙄)

Disabling them is not enough! You must uninstall the old version!

https://docs.google.com/spreadsheets/d/1mVFkj_7ij4FLzKs2QJaONNmb9Z-SRqUeG6xFGqEX1ew/htmlview

Also make sure to use the Compatibility Report mod! https://sp.zhabite.com/sharedfiles/filedetails/?id=2881031511

Issue: Traffic only uses one lane.
The Traffic AI is not broken! You just build your roads wrong.

Vehicles take the shortest, most direct route to their destination. That's how they've been programmed. They check each lane to see which one leads directly to their destination, and they avoid lanes that don't.

If you have a 6 lane road where everyone wants to go right, they will all take the right most lane because that lane is the most direct.

They choose based on pure, simple, cold maths. Not emotions, personal preferences, or which lane looks emptier or faster. They're computer bots, not humans.

If your six lane road only has one entrance into one neighbourhood, then everyone who wants to go into that neighbourhood will use the lane closest to that side of the road. You’ll need to plan accordingly.

6-lane one-way roads are complete overkill. The game has an active vehicle limit of 16.384. You will never have more than about 16k vehicles on your roads. At no point will those require 6 lanes in one direction. If they do, you need to redesign your roads.

Traffic Basics

• Always make sure you have at least 3 primary routes in and out of each area.
  
• Connect neighbourhoods to eachother, not just the main road.
  
• Highways are not for connecting neighbourhoods.
  
• Do not force an entire neighbourhood over a single intersection. It doesn't work in real life, and it doesn't work in the game.
  
• Avoid using more then 3 directions on an intersection. 3-way intersections (T and Y) flow better than 4-way(X and +). Don't ever make 5-way or more.
  
• Do not underestimate footpaths. Connecting neighbourhoods by footpaths can severely reduce road usage.
  
• Keep intersections at least 8 units (zoning blocks) apart. If it's a busy street, keep them at least 32 units apart.
  
• Roundabouts are not suited for high volume traffic. Keep them away from your highway entrances and busy streets!
  
• Use public transport. 30 Cims in a bus = 29 less vehicles on a road.
  
• Don't make your industrial traffic go through your residential neighbourhoods to get to their destination. Industrial <> Commercial <> Residential.

Public Transport basics

• Put stops on both sides of the street, and make lines go back the way they came. People have to be able to use the same route to get home.
  
• Connect areas to eachother, not just to the main station.
  
• Interconnectivity is vital! Allow Cims to transfer between lines and transit methods as often as possible!
  
• Shuttle lines from one end of the city to the other work best.
  
• Don't use circular one-way lines, they're useless.
  
• Think things through. Connecting a university to an industrial area is pointless.
  
• Nobody will use Public Transport if it takes 3 hours across the city to get to a shop that's 10 minutes by car!
  
• Use your brain!
  
• If you still can't figure it out, I'm guessing you've never set foot on a bus in your life. Leave your car in the driveway for a week. Do everything by public transport. Commuting, groceries, etc. You'll get it very quickly.

Help me fix my traffic!
Are you using the USA system of isolated neighbourhoods? If the answer is yes, take a good hard look at this image. Stare at it until you understand why your isolated, single entrance system is a terrible idea.

How do I manually change traffic lights stop signs?
They're under the Overlay tools. You don't need a mod or DLC for it, the functionality was added in the free 1.7 patch that everyone got.

How do I edit the length of the named roads?
Same place, third tab:

What are the official required hardware specifications?
Important: These are for the base game on release. Since then, updates that add content as well as DLC's have increased the RAM requirement. 6GB is not enough, you need 8!

Minimum:
OS: Microsoft Windows XP/Vista/7/8/8.1 (64-bit)
Processor: Intel Core 2 Duo, 3.0GHz or AMD Athlon 64 X2 6400+, 3.2GHz
Memory: 4 GB RAM (Not enough anymore!)
Graphics: nVIDIA GeForce GTX 260, 512 MB or ATI Radeon HD 5670, 512 MB (Does not support Intel Integrated Graphics Cards)
DirectX: Version 9.0c
Network: Broadband Internet connection
Storage: 4 GB available space

Recommended:
OS: Microsoft Windows 7/8 (64-bit)
Processor: Intel Core i5-3470, 3.20GHz or AMD FX-6300, 3.5Ghz
Memory: 6 GB RAM (Not enough anymore!)
Graphics: nVIDIA GeForce GTX 660, 2 GB or AMD Radeon HD 7870, 2 GB (Does not support Intel Integrated Graphics Cards)
DirectX: Version 11
Network: Broadband Internet connection
Storage: 4 GB available space

Can I run the game on an IntelHD Integrated GPU?
As listed in the specifications, Cities Skylines does not support Intel Integrated Graphics Cards. It might run, but it won’t run smoothly. Integrated GPU’s are meant for office use. Yes, that includes a Mac.

How many assets can I install on [xx]GB RAM?
It depends on the size of the assets. The game runs on the Unity engine, which loads all assets directly into the RAM on game load. so a 60MB train station uses 60MB RAM. A 10KB prop only uses 10KB, and so you can subscribe to a lot more before reaching the limit.

The average rule of thumb for the base game is:
4GB: 0-10 assets
8GB: 100-400 assets
12GB: 400-800 assets
16GB: 800-1200 assets
24GB: 1200-2000 assets
32GB: 2000-2800 assets
64GB: 2800-6000 assets

If it crashes on the loading screen, you've got too many. Remove them or buy more RAM.

What kind of hardware do I need to achieve different levels of modding the game?

There are roughly four different levels of playing: Vanilla, Semi-Modded, Mostly-Modded, and Detailer. Each has a very different level of system requirements. I'll be breaking it down by screenshots, so you can get an idea.

Note: I'm going by recommended specs, not minimum requirements. You might be able to do it with less, or might need more. It isn't an exact science, just a rough guideline

Also, I will be using Intel/Nvidia as a baseline, since I'm not familiar enough with AMD's current catalogue. You can find the AMD equivalent of any of these by looking on https://www.userbenchmark.com/

Warning: Integrated graphics chips such as the IntelHD and Intel Iris are NOT Graphics cards, and are not supported. This includes the ones in most Macbooks!

(Image source: Colossal Order)

Also known as the basic game without modifications.
The developers recommend the following official specs, which are pretty accurate.

Recommended:
Processor: Intel Core i5-5th gen or higher
Memory: 8 GB RAM
Graphics: nVIDIA GeForce GTX 660, 2 GB (Does not support Intel Integrated Graphics Cards)


This is the base game with a few custom assets and a handful of mods. Still everything zoned, a bit of RICO.

Recommended:
Processor: Intel Core i5-7th gen or higher, or an i7 6th gen or higher.
Memory: 12 GB RAM
Graphics: nVIDIA GeForce GTX 940 (Does not support Intel Integrated Graphics Cards)


Large parts of the city made up of custom content, handplaced with RICO, some detailing, but still some vanilla assets.

Recommended:
Processor: Intel Core i7 6th gen or higher.
Memory: 16 GB RAM
Graphics: nVIDIA GeForce GTX 1050 (Does not support Intel Integrated Graphics Cards)

.
This is what you see most Youtubers do as well.
No vanilla assets, 100+mods, thousands of custom assets and props. Everything handplaced with Plop The Growables and moved into place with Move It. You need a powerful system for this!

Recommended:
Processor: Intel Core i7 7th gen or higher.
Memory: 32 or 64 GB RAM
Graphics: nVIDIA GeForce GTX 1070 (Does not support Intel Integrated Graphics Cards)
Harddrive: SSD (For load speed. If you don't mind waiting over half an hour to load, a HDD is fine.)

Add (without the quote marks) under Steam > library > rightclick on Cities Skylines > Properties > enter it in the text field under Launch Options.


Load the game without workshop content:
Add '-noWorkshop' (without the quote marks) under Steam > library > rightclick on Cities Skylines > Properties > enter it in the text field under Launch Options.

Load the game without mods, but with custom assets:
Add '-disableMods' (without the quote marks) under Steam > library > rightclick on Cities Skylines > Properties > enter it in the text field under Launch Options.

Run the game in Directx9 mode (Windows):
Add '-force-d3d9' (without the quote marks) under Steam > library > rightclick on Cities Skylines > Properties > enter it in the text field under Launch Options.

Run the game in OpenGL mode (Mac):
Add '-force-glcore' (without the quote marks) under Steam > library > rightclick on Cities Skylines > Properties > enter it in the text field under Launch Options.

Developer console:
Add '--enable-dev-ui' (without the quote marks) under Steam > library > rightclick on Cities Skylines > properties > Properties > enter it in the text field under Launch Options.

Then press Tab while in game to open the console.

Windows:
Saves: %localappdata%\Colossal Order\Cities_Skylines\Saves
Screenshots: %localappdata%\Colossal Order\Cities_Skylines\Screenshots
Local Content: %localappdata%\Colossal Order\Cities_Skylines\Addons
Workshop content: C:\Program Files (x86)\Steam\Steamapps\Workshop\Content\255710
Main Game Directory: C:\Program Files (x86)\Steam\Steamapps\Common\Cities_Skylines

Copy the adress to Windows explorer. Or manually navigate to them. The Saves and Screenshots folders are in C:\Users\[Your Username]\AppData\Local\Colossal Order\Cities_Skylines. The Appdata folder is hidden by default, enable "Hidden Items" in Explorer to show the folder.

Mac:
Saves: ~/Library/Application Support/Collosal Order/Cities_Skylines/Saves
Screenshots: ~/Library/Application Support/Collosal Order/Cities_Skylines/Screenshots
Workshop content: ~/Library/Application Support/Steam/Steamapps/Workshop/Content\255710
Main Game Directory: ~/Library/Application Support/Steam/Steamapps/Common/Cities_Skylines

The Library is hidden by default, Under "Show View Options", make sure "Show Library Folder" is checked.

Managing assets
Steam stores workshop content where it's installed, and stores them inside folders by each game's Steam ID. Cities Skylines' Steam ID is 255710, hence the folder name for it is 255710
Windows: C:\Program Files (x86)\Steam\Steamapps\Workshop\Content\255710
Mac: ~/Library/Application Support/Steam/Steamapps/Workshop/Content/255710

Once in there you'll find a lot of folders with random numbers. These are the Steam ID's. Each workshop item has it's own Steam ID, like an address.

If you look at the url of any workshop item, you'll see a string of numbers at the end. For instance, The Ancient Shores is http://sp.zhabite.com/sharedfiles/filedetails/?id=1128355954

Those numbers after id= is the Steam ID, and that is the exact name of the folder The Ancient Shores has put all its files in: C:\Program Files (x86)\Steam\Steamapps\Workshop\Content\255710\1128355954.

Unwanted props in a pack
If you download a pack, but only want one item from it, it's better for your RAM usage to remove the unwanted props completely.

Options 1:
Copy only the .crp files of the ones you want from the asset's Workshop folder to the local assets folder. Then unsubscribe from the pack in the workshop. The asset will not be updated when the one in the workshop is updated.

Windows: From C:\Program Files (x86)\Steam\steamapps\workshop\content\255710\[Workshop ID] to %localappdata%\Colossal Order\Cities_Skylines\Addons\Assets.
Mac: From ~/Library/Application Support/Steam/Steamapps/Workshop/Content/255710 to ~/Library/Application Support/Collosal Order/Cities_Skylines/Addons/Assets

Options 2:
Simply delete the .crp files of the ones you don't want from the first folder. In this case they'll be re-downloaded every time the game is revalidated, Steam updates itself, or the asset is updated.

What are the sizes in the game?

• One unit = 8m x 8m; which is 64m²
  
• 1km = 125 units.
  
• One map tile = 1920m x 1920m; which is 3.686.400m² or 3,6864km² (~3,7km²), and contains 240 x 240 units, or 57.600 units in total.
  
• 9 tiles = 5760m x 5760m; which is 33.177.600m² or 33,1776km² (~33km2)
  
• 25 tiles = 9600m x 9600m; which is 92.160.000m² or 92,16km² (~92km²)
  
• 81 tiles = 17280m x 17280m; which is 298.598.400m² or 298,5984km² (~299km²), and contains 2160x2160 units, or 4.665.600 units in total.

This list uses the European Mainland decimal system (comma is decimal separator, dot is thousand separator.) For other parts of the world, just swap the dot and comma.

For more ingame sizes, check: https://cslmodding.info/scale/

What are the build limits?
The game has limits for everything, from buildings to trees to roads.
You can use the CSL Show More Limits mod to see all the game's limits, and how close you are to them.

The Trees limit can be extended, and the Area limit can go up to 81. All other limits are fixed, and can not be extended at present.

This is the complete list as per 5 November 2017:

What is Vanilla
Vanilla is a software term used to describe the original state of the software.
In the case of videogames, this means the game as it was made by the official developers.
DLC made by the official developers is also considered vanilla.
Vanilla Mods are the mods that are made by the official developers that come with the game.

What is Custom Content
Custom Content is everything for the game made by people who are not the official developers. You can find more information on custom content, and how to use it in your game, in the Cities Skylines | A Beginner's Guide To Using Mods & Custom Content. https://sp.zhabite.com/sharedfiles/filedetails/?id=1411897315

Is the Console version the same as the PC?
No. They removed the entire custom content support and the UI from the console version. The Xbox and PS4 version is a lite version of Cities Skylines.

Why is the Switch version not getting updates or DLC anymore?
On 11 June 2019, Paradox announced that the Nintendo Switch version is no longer supported. This is because of the extremely limited hardware of the Switch. The game has become too heavy for it. More here in the Official Announcement.[forum.paradoxplaza.com]

Can I use mods on consoles?
No. The Xbox has some custom assets that were added by the developer, but Microsoft does not allow mods. Sony doesn't allow any custom content at all. Neither does Nintendo.

Will the console version ever get mods?
No. They stripped the entire functionality out of the game when they ported it for consoles. The Steam Workshop is PC only, mod creators aren't going to adapt their mods for the console OS, and Sony and Microsoft don't allow third party content access to their system. The Xbox has some assets, but that's it. More will probably not be added because the consoles don't have enough RAM for it.

Is the Microsoft Store version the same as the Steam version?
No. The Microsoft Version is the Xbox version ported to PC. It still uses the Xbox interface, and has no custom content support. Do not buy this version for PC!

Can I use the Steam Workshop with the Origin version?
No. You'll have to buy the game on Steam.

Can I swap saves between computers?
Yes, that's possible. Just copy the .crp files from the Saves folder on the original system to the Saves folder on the destination system
Windows: C:\Users\[Your Username]\AppData\Local\Colossal Order\Cities_Skylines\Saves
Mac: ~/Library/Application Support/Collosal Order/Cities_Skylines/Saves

Screenshots
The game has its own Screenshot tool. You don't need to use Steam to take screenshots!
You can take a screenshot with F12.
Shift+F12 takes a screenshot with x4 resolution. (Does not work with Dynamic Resolution and PRocedural Objects mods.)
Screenshots are stored in:
Windows: %localappdata%\Colossal Order\Cities_Skylines\Screenshots
Mac: ~/Library/Application Support/Colossal Order/Cities_Skylines/Screenshots

Incidentally F12 is also the key for Steam Screenshots, but those are of a lower quality than the ones made by the ingame tool.

You can also use the basic oldschool Windows method of Windows key+Printscreen to save a screenshot of your entire screen automatically, you'll find those in C:\Users\[username]\Pictures\Screenshots

Terrain.party doesn't work. Are there alternatives?
Terrain.party often stops working, although it has now been down for several months. We may have to accept that it is dead.

Here are some alternatives. I can't vouch for how useful they are, though.
https://earthexplorer.usgs.gov/
https://gdex.cr.usgs.gov/gdex/
https://tangrams.github.io/heightmapper/

Moving to a different computer
I would always recommend doing it manually, the Steam Cloud has a habit of corrupting saves, and it doesn't copy your mod settings (including Prop It Up settings)

You'll need to copy three folders:
%localappdata%\Colossal Order\Cities_Skylines
C:\Program Files (x86)\Steam\steamapps\common\Cities_Skylines
C:\Program Files (x86)\Steam\steamapps\workshop\content\255710

Install Steam on the new system, but don't launch it yet.
Copy over those folders.
Then start Steam.
In the library, rightclick on Cities Skylines > Properties > Local Files > Reverify integrity of files.
This will detect the game as installed, (it may re-download some files) with all your mods and assets already installed as they were. (Which also prevents the "Workshop content won't download" problem)

You can also do this with any other games. This also saves time and bandwidth, as you don't have to re-download the full games. Just a few files.

How do I unsubscribe from everything?
This works best in the Browser version of the Workshop.

1. On the main page of the Cities Skylines Workshop, click on Browse.
   
2. In the drop down menu that appears, click on My supscriptions.
   
3. In the My subscriptions page, click on Unsubscribe All to unsubscribe from all Cities Skylines Workshop content.
   
4. If you have a lot of content, this may take a while.

How do I reset the game to a completely clean install?
Backup everything you don't want to lose, such as saves or screenshots.
Note that this method is for Windows, but you can use a similar method for Linux and OSX, the folder paths are just different.

1. Do the above steps for unsubscribing all Workshop content.
   
2. Uninstall the game. Open the Steam client, rightclick on Cities Skylines > Manage > Uninstall.
   
3. Delete the following folders via Windows Explorer:
   • %localappdata%\Colossal Order\Cities_Skylines
     
   • C:\Program Files (x86)\Steam\Steamapps\Workshop\Content\255710
     
   • C:\Program Files (x86)\Steam\Steamapps\Common\Cities_Skylines
4. Open the Steam client and reinstall the game.

Some of these have already been mentioned in the Guide, here's a recap and some additional ones:

Avanya's Must Have Guide: http://sp.zhabite.com/sharedfiles/filedetails/?id=823267366
HOW TO TRAFFIC by Soushi: http://sp.zhabite.com/sharedfiles/filedetails/?id=410236188
Loading Screen Mod: https://sp.zhabite.com/sharedfiles/filedetails/?id=2858591409
CSL Show More Limits: http://sp.zhabite.com/sharedfiles/filedetails?id=531738447

Official Cities Skylines Forum: https://forum.paradoxplaza.com/forum/index.php?forums/cities-skylines.859/

Cities Skylines Reddit: https://www.reddit.com/r/CitiesSkylines/
Gaming Tech Support Reddit: https://www.reddit.com/r/pcgamingtechsupport/
Tech Support Reddit: https://www.reddit.com/r/techsupport/
Can I Run It Reddit: https://www.reddit.com/r/CanIRunIt/
Suggest-A-PC Reddit: https://www.reddit.com/r/suggestapc/
Suggest-A-Laptop Reddit: https://www.reddit.com/r/SuggestALaptop/

If you're looking for a friendly Cities Skylines community, come join us over at the SkylinesNation Facebook group. Beginner or master, creator or player, PC or console, everyone is welcome! (We do NOT do Tech Support!) https://www.facebook.com/groups/SkylinesNation/