Just released an update that fixes the stairs and makes the ladders a bit easier to climb. The one-entrance upper floor felt okay from what I've played of the map since there's so little space and there's like 3 Infected spawns up there, but if it ends up being a problem even after fixing the stairs I can try and do something to balance it.

Stairs don't work and ladders are difficult to use. One of the upper floors only has one entrance (the broken stairs) and would be kinda unbalanced even if they worked.