Sigil

Sigil rests on the inside of an open torus, like a bicycle wheel with no inner tube, around the top of an infinitely tall spire in the middle of the True Neutral alignment plane and literally at the center of the multiverse. 

The torus is a few miles across and gravity pulls everyone “down”, towards wherever the ground is closest to them. So if you look “up”, you see the spire and then open air, and then the city on the inside of the other side of the ring. Sigil does not rotate around the spire, it just sits, and there is no sun or moon. The air itself provides ambient light which waxes and wanes over a 24 hour cycle.