Okay, Sigil is not going to play a large role in this, just an exotic location. So don't worry about factions and being largely ignorant of Planescape.
As a little background: It is impossible to Gate, Planeshift, or otherwise go to Sigil, except through one of the naturally occuring Gates that exist all over the place. It is a difficult (DC 30) KS: Planes check to know where one is, but it is realitively easy for you guys.
Sigil is not the place to flex your destructive capabilities, by the way. Sigil's best known inhabitant is called the Lady of Pain and she has no stats and is a big floating plot device that keeps Gods out of Sigil and prevents people from being overly destructive in her city. She is otherwise an enigma. Appearing and disappearing at seemingly random.
Finally Sigil sits on top of the infinitely tall spire that sits in the center of the multiverse. (How something that is infinitely tall has a top and how something infinitely big has a middle are subject best not contemplated.) It is built on the inside of a gigantic torus (Donut like shape). It is a bizare place full of interesting "people."
