Ok, so it can be done on levels. Weeee.!.!.!

But take a look at the structures of ESIV:Oblivion. All buildings
can be entered. You think their insides are also random or they
did it "ONE BY ONE"?Im thinking its random 'cause the whole of oblivion has about (i think) 200+ buildings.

Still we have to accept that randomization add up a little replayability to
a game. Without it, the game's boring.