"You look lost, sirrah. Walk with me through the murky twilight for a moment, and I'll teach you the dark of things - thad' be the secrets, for a clueless traveler like yourself. Mind you don't brush against the grime; there's lots of soot-stained walls here. Now, sir prime, look up. Makes you dizzy, don't it, seeing the city of Sigil above you? See, living in an impossible city ain't always simple. . . "
The heart of the Outlands, and therefore the self-proclaimed center of the planes, Sigil is known as the City of Doors. Portals leading throughout the cosmos lace every district of the city. Situated atop the spire itself that rises above the surrounding plane, Sigil is a torus floating in space, with the city itself constructed along the inside of the band.
Sigil is proof against deities. Whether because of the Lady of Pain's desire, Sigil's place atop the Spire, or an agreement among the deities, deities do not enter Sigil.Nevertheless, the city has a huge number of portals, the sum total of which is unknown even to the inhabitants. There are portals to every known Outer Plane, every Inner Plane, and even (it is rumored) gates to alternate Material Planes. Portals also connect Sigil to other locations in the Outlands, or other places within the city itself.Sigil is a merchant's city. Goods, merchandise, and information come to it from across the planes. There is a brisk trade in information about the planes, in particulre in the command words or items required for the operation of particular portals. These portal keys are sought after, and usually travelers within the city are trapped, looking for a particular portal or key to allow them to continue on their way. The city is controlled by a number of factions, all of which may be politely described as "philosophers with clubs." These factions control different parts of the city and the services therein. But the ultimate ruler of the City of Doors is the enigmatic being known as the Lady of Pain. The full extent of the Lady of Pain's power is unknown, but it is widely assumed that her power equals or exceeds that of the deities themselves.