Every old, abandoned, locked door leads inside a temple of varying heights (the first one I went into was 36 - 7 floors, maybe only 6, the last was 10) that serve as gateways to other worlds. I found this out by accident, the door I opened on the other side led to a world of Nazis. They heard me and came looking for me. The bad thing about these temples is that, apparently, once a door is opened it can't be locked (or even properly closed) again. I ran out of the temple, kept thinking about locking the door somehow, but never did. Two years passed in a matter of minutes (the way they do in dreams) and armed people came out of the door.