Func ladder

It creates a Half-Life style ladder which can be climbed on vertically or horizontally.

SDK source code mentions this entity being converted into one called, however neither CS:S nor DOD:S implement that entity (but still attempt to create it at runtime). The brush is only climbable due to the fact that VBSP also adds to the BSP flags. The Left 4 Dead games do have  in the engine, but instead use  for the classname.