Originally posted by fantasyze:

you'll find them wherever you find skeletons in the black forest. just keep exploring until you see skeletons

Sort of.
Skeletons inhabit ruined towers which do NOT have a crypt at them (in general, could be one near by coincidence). Skeletons also can roam near graves (nearby meadows) or other biomes (nearby swamp, etc).

Regardless, skeletons are still your best indication to find them, just give it a sanity check that its not the border with another biome or part of a ruin or something else. The entrances here are large, you really can't miss it if its there and you found the guards.