Sigil is a massive extraplanar metropolis that traditionally exists in the infinite sky of the Outlands, the True Neutral Plane, floating atop the infinitely tall Spire at the centre of the Outlands. As its moniker of "the City of Doors" suggests, Sigil is absolutely lousy with portals throughout the Multiverse, allowing access to theoretically every single planar location and prime world that exists. This makes it the grand melting pot of the cosmos, an interdimensional port-city where everything ends up going to and eventually ends up coming from.
Sigil is constructed in a torus shape, which is surprisingly frustrating to describe. Essentially, picture a giant tube, with a city built on the interior of that tube. Then join the tube's ends to form a single unbroken circle. From the outside, Sigil looks like a giant ring.
Sigil is ruled by the enigmatic Lady of Pain, a godlike entity who has but a single law; Don't Fuck My City Up. Actual day-to-day operations are left to the inhabitants to oversee.
Sigil's native denizens, known as Sigilies, are somewhat infamous for their arrogance, sort of the stereotypical Big City Slicker looking down on the rural country bumpkins - which, in their mind, is pretty much everyone who doesn't live in Sigil, but especially Prime Material denizens. This does occasionally bite them, such as when a "Clueless" turns out to be an epic tier wizard who promptly vaporizes the tiefling street gang trying to mug him.