The Abyssal Situation
The City of Doors
Sigil is located atop the Spire in the Outlands. It has the shape of a torus; the city itself is located on the inner surface of the ring. There is no sky, simply an all-pervasive light that waxes and wanes to create day and night. Sigil cannot be entered or exited save via portals; although this makes it quite safe from any would-be invader, it also makes it a prison of sorts for those not possessing a portal key. Thus, sometimes Sigil is called “The Cage”. Though Sigil is pseudo-geographically located “at the center of the planes” (where it is positioned atop the infinitely tall Spire), scholars argue that this is impossible since the planes are infinite in all dimensions, and therefore there can never truly be a center to any of them, let alone all of them; thus, Sigil is of no special importance. Curiously, from the Outlands one can see Sigil atop the supposedly infinite Spire.
Sigil contains innumerable portals that can lead to anywhere in the Dungeons & Dragons cosmology: any bounded opening (a doorway, an arch, a barrel hoop, a picture frame) could possibly be a portal to another plane, or to another point in Sigil itself. Thus, the city is a paradox: it touches all planes at once, yet ultimately belongs to none; from these characteristics it draws its other name: “The City of Doors”.