It was located in a cave system in a densely wooded area of Harlan's World, near to a beach that Takeshi Kovacs and Reileen played on as children. A massive Songspire Tree grew in the middle of the cave, towrds an opening in the ceiling. It housed several dozen of Quell's followers and their families.
Owing to the betrayal of Reileen, the inhabitants of Stronghold were infected with the Rawling Virus, while the woods around the cave system were firebombed, filling the caves with heavy ash. This lead to the Battle of Stronghold where CTAC wiped out the Envoys.
There was a lower cave system that housed the root system for the Songspire Tree. It was here that Reileen hid the re-sleeved Quell in a "coffin" and tortured her in Virtual reality.