The Gatehouse is Sigil's asylum for the mentally ill in the worst part of the Hive Ward and was the headquarters of the Bleak Cabal prior to the Faction War.
Once it might really have been a gatehouse for a massive wall built within the City of Doors - it certainly looks like one. However extensive the wall might have been, only the Gatehouse is left.
Source
The Factol's Manifesto
- 727 reads