Just found that out a few minutes ago. Now I'm trying to get into the bottom-left room of floor B1. I've opened every other door in the dungeon except the boss room and the room right of where the compass was. I know that I must get into the bottom-left room of Floor B1 by taking the North entrance to the room next to it, and I've already opened that door, but I can't get to the north entrance. There's a bridge that I'd be able to take, but the middle is out.