If I recall correctly, Planeshifting and Summoning don't work in Sigil.
What about Dismissal and Banishment? Can you unsummon a fiend back to Hell if it stands in Sigil?
Is there any way to move into and out of Sigil except by Portals?
On a related note, in my campaign I have ruled that the Ethereal and Shadow planes co-exist with Sigil but don't extend beyond it. So movement through those planes is possible in Sigil. Also, you can teleport or dimension door within the city, but not to anyplace outside of it.
Thoughts? Rules? Suggestions?
I am pretty sure that banishing somebody outside of Sigil is just as impossible as summoning them in. Everybody except for the Lady and her dabus (or the dabus and their Lady) are extraplanar in Sigil, so there is doubtlessly a reason for it. Maybe, just maybe, that is one of the reasons that the Lady mazes people in bits of Sigil rather than sending them outside.