The one found just before the very end, in a hallway on a table. Just hold alt, and you should see it easier. Look for "iron bar". I haven't tried it, but I suppose it can be used to block the wheels of the gates for the spiders, to trap them inside.
Better grab the claymore and smash the spiders for experiance. Plus you get a key in one of the haystacks.
A good RPG has several ways to deal with problems. That was a proof that BD is definitly a good RPG <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />.