Sigil

Sigil, called the City of Doors and the Cage, is the city located (presumably) atop the infinite Spire in the center of the Outlands. It is ruled over by the Lady of Pain, a frighteningly powerful and mysterious figure capable of controlling the city's portals and preventing Powers from entering the city.

The city is said to be the fulcrum point for all of the planes, and as a result whoever can control the city controls the multiverse. This is why the city up until recently has been run by competing Factions. Each faction believed that if it controlled Sigil, the Center+of+All, it would be able to control everything there is. As a result, the Lady of Pain kept the Factions (and many other power brokers in the city) in balance.

Sigil is shaped like a torus, or a tire, curved both longitudinally and latitudinally but open along the inner circle so that the other side of the city is visible in the "sky" It is utterly dependent on the portals for all trade, air, water, and resources. The city is often cramped, though at the Lady's whim streets and buildings will occasionally shift, appear, and disappear. The air is often polluted, and acid rain is a problem in the city again because of the lack of easy air flow. Sigil does not recieve any true sunlight, so without special assistance plants are difficult to grow. Razorvine is a notable exception to this rule as it can be found growing at tremendous rates throughout the city. The city is tended to by the Dabus, who repair buildings and trim the razor vine.

There is no 'outside' of Sigil and it is sealed against teleportation, planar spells, or any other work arounds to enter the city including epic magic. It's portals or nothing. While it is said to be atop the Spire (and some claim to see it hovering there on a clear day), there has never been a successful attempt to climb into Sigil from below. In a simular fashion no one - or almost no one - has ever returned from jumping out of the 'side' of Sigil. The side of Sigil appears to lead nowhere, and those foolish souls who jump may fall forever, become erased from existence, or end up in an entirely random plane.

Additional Resources
Brix's Guide to Sigil