Making a Ladder

First off create your walls/ledges or mountains where you want ladders to be next to.
There are 2 ways to make a basic ladder for CS:S

First one is to make a brush and apply a nice ladder texture onto the brush(you can browse for textures *filter word "Ladder")
(align texture using the texture application tool).Then hit the "To Entity" button and make the
brush a func_ladder entity. Here are some screenshots of the ladders in game.

The second version of the basic ladder is almost the same as th first. Instead a brush based ladder
it will be a model based ladder.

Now hit the entity tool and pick "prop_static" from the list of entity's. Hightlight World Model
line and browse thru models for a ladder. (some can be found in the props/c17 folder)
Model List Link
Move the ladder model around in 2d views and line up to wall, Now make a brush about the size
of the model and cover it in the nodraw texture and turn this brush into the func_ladder entity.
Then line up func_ladder brush in front of the model.
(see Images below)

also you can use the ladder texture and make a brush, compile and it will be a climbable ladder
Now you have a climbable ladder