Start off with the area where you want to have the ladder. This will be a simple climbable wall.
I cut out a wall and textured it, duplicated the face, made it a bit smaller (in this case its too small, just to show the texture behind it), textured this dupe as "trigger.png" and set appropriate properties (as on the pic to the right), made my grid smaller with (']' key) and moved it just one unit in front of my textured ladder. Finished ladder below.

In this setup, however, when the player walks up the ladder, he can do a super jump (like on regular jump pads, if you hold down your jump key when bouncing off jump pads). To prevent that, we can lower the trigger face a few units AND align the top 2 vertices with our textured ladder (forming a small triangle between the trigger, ground and the backwall), make sure to test it thoroughly so players wont take shortcuts through levels :)

Our previous ladder was no wonder, it works but looks lame. You can do something as simple as creating some vertical walls on each side of ladder, stretching the trigger all over the ladder too, like on the image above. This already looks and works better. If you fancy, you can make it look pretty by making steps out of faces (or even creating a model and importing it as object!). It is up to you how you will design it, I like to have the steps passable so when climbing, you are "restricted" within the ladder (just make sure to make the steps passable for player/monsters). Example can be found in my map Foundry (the trigger face is at the back of the ladder, the pink paintover on the image below).

You can download it here from my Dropbox[dl.dropbox.com].