In the
Planescape series of
novels,
RPGs and
computer games Sigil, also known as The Cage or The City of Doors is a
city often refered to as being the crossroads of the
Multiverse. Physicaly the city is on the inner face of a
torus (it's on the inside ring of giant
donut) and appears to float hundreds of miles above the spire in the centre of the
Outer Plane known as the
Outlands. The city does not spin in the style of an
Arthur C. Clarke spacestation, things just naturaly fall outward: the laws of physics are not entirely as we know them. A side affect of this set up is that if you look up you will see the streets and houses sweeping up, and over your head. Vomiting is a common reaction to a first glimpse of the fair city.
The only way into or out of the city is through the interplanar portals which dot the city. Nothing else, magical or technological, will allow entrance or exit. The profusion of portals are the reasion for the prosperity of the city, you can get anywhere from here and you can ususualy find someone who knows where to find the right portal. The result of this is that much trade and traffic passes through the city.
Added to the convienience of the portals is the fact that the Lady of Pain (an enegmatic and secretive figure) controls everything including the portals, and has decided the city will be neutral ground; no one force may rule Sigil, anyone who tries vanishes...if they are lucky. This means that Tanar'ri and deva, elves and orcs can all visit Sigil to trade or negotiate in the knowledge that it is neutral ground. This is not to say that the Lady is a political ruler, in fact she is hardly seen at all and reports of her actualy speaking are generaly confined to dark nights beside the fire after attacking the ale. If you meet the Lady on a visit to Sigil (you'll recognise her; she's about ten feet tall, floats about five feet off the ground and has a face surrounded by blades) don't talk to her, don't ask her for help and whatever you do don't attack her.
The political wheeling and dealing in Sigil is done by the Factions, essentialy guilds based around philosophies and views of the Multiverse. These groups provide what passes for a governmental structure for the city; things like a police force (the Harmonium), Courts (the Guvners) and a Mortuary (the Dustmen). Most Planewalkers belong to one of the Factions, though this is not mandatory.
A Potted History of the Idea of Sigil
Sigil was first hinted at in the AD&D 1st Edition Manual of the Planes but was only realy came into being with the advent of the AD&D 2nd Edition Campaign Setting called Planescape. This setting, while being marketed as opening to doors to the infinite Multiverse in fact was largely based in the city of Sigil, providing a very detailed street by street picture of this chaotic melting-pot.
With the advent of Dungeons and Dragons 3rd Edition (d20) a new version of the Manual of the Planes was released. This went back to the 1st Edition Manual for feel and outlook but drew on the rich resources created for the Planescape setting. In the new Manual the focus shifted away from Sigil back to the infinite Planes and the infinite scope for adventures that they present. Sigils position was presented as not the be-all-and-end-all of Planar travel but as a valuble nexus of plot-hooks and a jumping-off point for interplanar adventures.
The idea of the city at the centre of the Multiverse is common through much of the genre, similarities in concept exist with Amber and Tanelorn, though it must be said that Sigil is definitely much more alive than eaither of those cities. Though Tanelorn sounds a much nicer place to live ;-)