Making a ladder or climbable brush

1. Firstly make your ladders side beams. Ive textured mine with the texture "ametal_m01" from the "metal misc" texture set, as you can see below:

2. Make the ladder foot holds with brushes or an alpha texture

3.Now make a brush where you want the ladder to be climbed, also include the side beams of the ladder and make the "ladder" texture a few units thick, like in the following images:

Yellow = Side beams of the ladder
Pink = ladder_dark texture from the alpha set
Aqua = ladder texture from the common folder

4. You should now have a working climbable ladder, if you wanted to make other things climbable such as fences, bushes etc., just place the "ladder" texture from the "common" texture set where you want to climb up, if the brush is made too thin it might not be enough for the player to climb up, too thick and it might look like the player is a long way from the ladder, so take care and be sensible.