Sigil

The City of Doors.

Sigil is a plane of portals that connects to other planes (and to itself) through hundreds or thousands of portals throughout the city. The city itself is torus-shaped. One can stand on the street and look up and see buildings looking down above them hanging like stalagtites.

Sigil floats above a tall spire in the center of the Outlands. It is a place of true neutrality. An ominus figure named the Lady of Pain controls the city and has the ability to negate magic and even keep gods from entering Sigil.