I seem to have gotten stuck on on the fourth level or so (whenever the wall textures first change). I can only explore the upper left quadrant of the map, and t

I seem to have gotten stuck on on the fourth level or so (whenever the wall textures first change). I can only explore the upper left quadrant of the map, and there are no doors or stairs in the accessible area. I've also bumped into all of the exterior walls, in hopes of finding a hidden passage. Help?