From the introduction:
It’s considered by some to be the jewel of the Outlands, the nexus of power of the multiverse, a place of great beauty or ugliness. Every day in Sigil, deals are made that seal the fate of hundreds and thousands, negotiators barter for peace in the Blood War, and factotums seek the truth to the meaning of everything. People die, ideals are born, and great power is wielded with the flick of a finger.
Sigil is a place of contradictions. It’s the center of an infinite multiverse. It’s an embattled neutral ground in a battlefield of philosophy. It’s the City of Doors and yet it’s called the Cage. It’s home to pit fiends and solars, devas and yugoloths, all living and drinking and socializing in more or less peaceful state.