Manual of the Planes 3.0
Quote:
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 ring floating in space, with the ity itself constructed along the inside of the band. (...)
There is more in there, but I do not want to copy type the whole entry. In addition it is not very detailed, as was mentioned above. Though if you really want to read up about it, find the two following books I listed bellow. Easy to be found as pdfs.
Planescape:
In the Cage - A Guide to Sigil
Uncaged - Faces of Sigil