func_ladder
func_ladder is a internal brush entity available in all Source games.
Note: This entity not avaliable since Source SDK 2006 and later and doesn't function in ep1-2 series, even if to add it to base.fgd. Use Func_useableladder instead. Note: This entity does not function in 
Counter-Strike: Global Offensive. Consult Creating Ladders (CSGO) for help.Entity description
A ladder. Players will be able to freely move along this brush, as if it was a ladder. If you are using a model prop for the visual representation of the ladder in the map, apply the toolsinvisibleladder material to the func_ladder brush.
This entity has no keyvalues, flags, inputs, or outputs.
In code, it is represented by class CInfoLadder, defined in nav_ladder.cpp.
Note: This is an internal entity. When the map is compiled by VBSP it is processed and then removed: it does not exist when the map is running.