Alterations to Magic in Sigil

Planar magic works differently in Sigil. The following magical restrictions apply there: 

Banishment

Effects that banish a target from Sigil treat the target as if Sigil were its home plane.

Extradimensional Space

Extradimensional spaces, demiplanes, and pocket dimesnions-- such as those created by a bag of holding or the rope trick spell-- function within Sigil, but those spaces follow all these restrictions as if they were part of the city.

Planar Travel

Effects that allow interplanar travel, such as astral projection or plane shift, fail if used to try to enter or leave Sigil, with one exception (see Teleportation circles below). 

Summoning

Spells, magic items, and effects that summon creatures or objects from other planes, such as a ring of djinni summoning, instead summon targets from within Sigil if possible or otherwise fail. Effects that summon a specific target from outside Sigil, as with a Leomund's secret chest spell, fail automatically.

Teleportation

Attempts to teleport into or out of Sigil fail, but such magic functions normally when used to teleport within the city. 

Teleportation Circles

Permanent teleportation circles exist within Sigil, but attempts to create new ones fail. Those with permission can use them to enter the city via the plane shift spell but not to leave.

Type

Metaphysical, Arcane