The City of the Saved was a metropolis the size of a spiral galaxy located between the end of the Universe and the beginning of the next. The City itself was the timeship Compassion, who had lodged herself between Universes. (Of the City of the Saved...)
It was inhabited by every single Australopithecine, human, near-human, and humanoid who was ever resurrected after death in an immortal, invulnerable body. Only human beings were resurrected and those that were partially human were treated as second class citizens known as "collaterals" who were also forbidden from holding public office.
Whenever history was rewritten by the War between the Great Houses and their Enemy then the City's history was retroactively rewritten with it to include any new human beings or versions of them that were brought into existence by the change. This meant that the population of the City was much larger than anyone else on the outside suspected.