Here is a description of Sigil from the "Manual of The Planes" which might shed some light:
Sigil
The heart of the Outlands, and therefore the selfproclaimed 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 ring floating in space, with the city itself constructed along the inside of the band.
Sigil has a number of special traits, one of which is objective directional gravity. “Down” is toward the ring itself. Those who escape the ring suddenly find themselves in open air, often plummeting down the side of the spire.
While magic is extremely limited near the spire, the City of Sigil has the normal magic trait. Again, once over the edge of Sigil's walls, the trait for the rest of the Outlands takes effect.
Sigil is apparently 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.
Sigil 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, usually the portal towns. These portals require command words or command items in order to function.
Sigil is a trader's city. Goods, merchandise, and information come to it from across the planes. There is a brisk trade in information about the planes, in particular 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 looking for a particular portal or a portal key to allow them to continue on their way.
Sigil is controlled by a number of factions, all of which may be politely described as “philosophers with clubs.” These factions break down along the lines of traditional alignments and control different parts of the city and different services therein. The ultimate ruler of Sigil is an enigmatic being known as the Lady of Pain, a floating female humanoid with blade like hair. The power level and abilities of the Lady of Pain are unknown, but it is widely assumed that her power equals or exceeds that of the deities themselves.
