From Ardrana

Sigil is a city located in the center of the Outlands and ruled by a mysterious figure called the Lady of Pain. It is, essentially, the crossroads of the multiverse, where people come from and leave for any and every conceivable location -- including Ardrana. There's a reason it's called the City of Doors -- there are portals to other places all over the city.

Sigil is built on the interior of a great torus, which is somehow held in place around The Spire. Thus, when one looks upwards -- on those rare occasions when the air is clear enough to see that far -- one can see another part of the city above them. The city is divided into six wards: the Clerk's Ward, the Guildhall Ward, the Market Ward, the Lady's Ward, the Lower Ward, and The Hive.

Note: The city of Sigil originally appeared in the Planescape AD&D campaign setting from TSR. Additional information can be found in In the Cage: A Guide to Sigil. Its use here is for the purposes of providing context for the campaign only.