Some dungeons weren't built by anybody. They weren't formed by natural or arcane processes. They grew. When enough people think some idea, there's a dungeon for it somewhere. When enough people feel one way, their feelings creep together and coalesce like a cyst underground, to give those feelings shape. Woe be to the adventurer that enters this sort of dungeon--unless, by chance, they recognize the feeling that gave it birth, and recognizing it, know how it may be slain.