Just killed the second of the three boneyard keepers - the one on the floating tower. Now I discover there's no exit from the tower. How do you get back to ground level after killing the keeper?

Go to the highest level, jump in a "fire ring" similar to the one which shot you up there.

Thanks. Found the "fire ring", tried it as you suggested. Nothing happened. Tried exiting and reloading the game, then jumping in again. Same lack of result. (And, yes, now is the time you can all laugh at me because I didn't have a save before I jumped onto the tower in the first place. I looked, and ALL my backup saves are on the floating tower.)

Have you got "become Ethereal" shout? If so use it and jump off the tower !

This absolutely worked like a charm. Looked pretty cool on the way down, too. Thanks!