"Sideney followed him reluctantly up the stairs.
The tower--if that's what it was, he thought; he was used to the odd architecture at Unseen University and this made UU look normal--was a hollow tube. No fewer than four spiral staircases climbed the inside, crisscrossing on landings and occasionally passing through one another in defiance of generally accepted physics. ... What threw the eye was the absence of shadows. You didn't notice shadows, how they delineated things, how they gave texture to the world, until they weren't there. The white marble, if that's what it was, seemed to glow from the inside. ..."
Hogfather by Terry Pratchett