Came across this excellent article about the Planescape world.
http://thewertzone.blogspot.com/2009/11/worlds-of-d-planescape.html

And here is a great picture of what the city of Sigil looks like from the outside.
(Inside of a torus, wtf? On which edge? OHH...) -
http://thewertzone.blogspot.com/2017/02/cities-of-fantasy-on-patreon-sigil.html