Sigil was introduced in the Planescape campaign as the "city at the center of the multiverse".
[....more stuff about Sigil here...]

The heroes who participated in the "Die, Vecna, Die!" adventure ended up in Sigil. After successfully driving Vecna from the city, the Lady of Pain gave them three rewards:

1. Vecna would not pay any attention to the characters unless they went out of their way to annoy him.
2. A key to the city of doors: a wondrous item that can be used to make three two-way, keyed portals between two locations of the user's choice, one of which must be somewhere in Sigil. The activation key is set at the time each portal is created and can be whatever the owner of the item desires.
3. The deed to a mansion in the Lady's Ward of Sigil.

Vandis, Veracity, Hazen, Kress, Igor, and Tumulo all moved into this mansion as their primary residence. Elvira has a suite of rooms in the mansion and is welcome to stay any time she wishes, but does so infrequently, as she presides over the Temple of Lei-Kung in Mansfield.

There is easy access between this mansion and Mansfield. Using the key to the city of doors, Vandis made a two-way keyed portal between Sigil and Mansfield.

If you are on Mansfield standing in front of Tebryn's tower and activate the key, the front door of the tower takes on a mirror-like sheen for one minute. During this period, if you walk through the portal, you emerge into the inner courtyard of the mansion, as if you had walked through the passage from the street.

Conversely, if you are in the inner courtyard of the mansion in Sigil facing the passage to the street, when you activate the portal, the archway takes on a mirror-like sheen for one minute. If you walk through the portal, you emerge in front of Tebryn's tower in Mansfield, as if you had just emerged from his front door.

Here is a tour guide of the mansion, with my maps of the floors inserted appropriately.