Planescape Part One

From the introduction:

Welcome to Sigil, the City of Doors! The centre of the Planes and the local multiverse too. Sitting on the inside of a giant torus that itself floats above an infinitely tall mountain at the centre of the very plane of neutrality itself – the Concordant Domains of the Outlands. Here neutrality is enforced. Gods are barred from the city, and the Lady of Pain takes very aggressive steps to make sure nobody drags Sigil into the troubles outside. It is even a haven from the Blood War. Unfortunately, this means many, many criminals and troublemakers from outside land come here as a last resort.