Any mob can climb a ladder in the same way a player does: by pushing against it. Mobs are not smart enough to deliberately use ladders — they won't stay on a ladder to get somewhere — but a ladder directly in their straight-line path will let them climb up
Sometimes well, I suppose it is actually quite useful when compared to the dwarf fortress wiki the only thing confusing in the wiki is the redstone section. You're much better looking at redstone tutorials.
For my sheep pen. Build a two block tall wall for the back of the pen, the rest can be fence. Have a step up to get on top of the wall, and on the other side leading into the pen, have a single ladder on the top block of the wall. Sheep can't jump up and climb the ladder, but YOU can. This will allow you to get into the pen, sheer them or whatever and then climb back out without them being able to follow you up.