Sigil, city with a hundred names: The City of Doors, the Hub of Existence, the Ultimate Price, the Cage, Sigil Bejewelled, soddin' Hole of Dirt and of course: "What?!"
Sigil lays in the center of the Outlands. Or at least a berk might think so, when he is looking from there. Hovering above the endless spire, a city in the shape of a 6 mile diameter torus. Of course, since the spire is infinitely high, no one has ever reached Sigil that way. Doesn't mean some barmies try it, of course.
Instead, the City is reached by it's distinguishing features: an endless supply of interplanar portals. In fact, every arch, every bounded space here can be a portal to another plane, be it one of the myriad primes, the inner, outer or transitive. Of course, every portal needs a key, and that is one of the reasons why it's called the cage. You ain't got a key, you ain't going. That simple, berk. In fact, not even highest-level magic will get you out of Sigil.
Why that is the case? Because of Her Serenity. The Lady of Pain.
She is the highest high-up there is in the City of Doors. She opens and closes the portals at her will and whim, changes their keys and destinations. And, of course, she's keeping the bloodwar and the powers out of here, which is very, very necessary. See, if she didn't do it, all the so-called gods and all the fiends would be crawling over the city. Control of this city means control over every planar trade-route and access to every place ever imagined. And that's not her only power. If you cross her, or worship her, or kill a dabus, or she doesn't like you... well, bad for you. She will either flay you with her shadow or maze you. Mazing? Oh, yes. See, the lady can control the city's shape and size. Therefore, if she wants, she splits part of the city off and throws it over to the ethereal, to serve as a prison.
Anyway. Enough about her, it's not wise to speak too much or too loud about Her Serenity.
The city consists of six wards. The hive ward, were all the poor sods live who have nowhere else to go, the lower ward, where all the craftsmen and industry are, so called for it's portals to the lower planes and it's population of fiends, the clerk's ward, self-explanatory, the market ward, home of the great bazaar, the guild hall ward, which is home to most of the craftsmen and the lady's ward, where the high-ups live. (Not the lady herself, though. No one's ever seen her "live" as such.)