"These boots have walked on a dozen worlds, these eyes have seen horizons beyond your imagining." -Quentin
Found on Remnant, the rogue plane, Sigil is a toroid with cityscape along the inside, balanced atop a seemingly-infinite spire and populated by creatures from every corner of the multiverse. Sigil is notable for a number of reasons, the first being that its position on Remnant, and the rogue plane's unique nature, makes the City of Doors the center of the multiverse. Secondly, nearly every opening in the city itself, be it a door, a window, or even a laundry chute, is a portal to somewhere else in the multiverse, just waiting for the right magic to open it.
Bards cannot agree on who built Sigil, or when, or why. The only reason storytellers even know that it is on Remnant is because mages from Rumor mentioned being able to see it from their academy windows. Stories that it was once the fortress of a cult seeking to break down the living gate remain popular, but so do stories that it was built as a vacation spot for a mad, rabbit-obsessed wizard.
During the War of Ruin, the wizards of the Spire altered their Cullis Gate network to route every teleportation through Sigil, which made it the most important asset to both the Empire of Bael Turath and the Empire of Arkhosia. It was the bloodiest battlefield of the war, contested even between the nine invasions. Bodies piled up outside houses, and the streets ran with blood.
In the third era, Sigil was mostly forgotten by the mortal realm, having fallen under the rule of the mysterious Lady of Pain. Sigil's ostensible government employed beings from across the planes indiscriminately, and it was not uncommon to see angels working alongside demons, devas with devils, or dray with men. In the fourth era, the only god Selesnya cut the mortal realm off from the rest of the multiverse, Sigil included. After her defeat, the New Host restored the mortal realm in the multiverse.