Planescape Canonical Campaign
At the center of the Outlands, like the axle of a great wheel, is the Spire — a needle-shaped mountain that rises high into the sky. Above this mountain’s narrow peak floats the ring-shaped city of Sigil, its myriad structures built on the ring’s inner rim. Creatures standing on one of Sigil’s streets can see the city curve up over their heads and — most disconcerting of all — the far side of the city directly overhead. Called the City of Doors, this bustling planar metropolis holds countless portals to other planes and worlds.
Sigil is a trader's paradise. Goods, merchandise, and information come here from across the planes. The city sustains a brisk trade in information about the planes, particularly the command words or items required for the operation of particular portals. Portal keys of all kinds are bought and sold here.
The city is the domain of the inscrutable Lady of Pain, a being as old as gods and with purposes unknown to even the sages of her city. Is Sigil her prison? Is she the fallen creator of the multiverse? No one knows. Or if they do, they aren’t telling.