Chronopolis

Chronopolis was a city created by the Valeyard in an alternate timeline, where that dark incarnation won the trial against the Doctor. Following his claim of the Doctor's regenerations, he achieved immortality. The Valeyard crafted a city within the Time Vortex around his TARDIS, which protected it from harm. The Valeyard placed a sample of every extinct species within. Chronopolis looked different to every viewer. (BFA: He Jests at Scars...)