Chronopolis

Chronopolis was a city created by the Valeyard. After gaining immortality, the Valeyard crafted a city within the Time Vortex around his TARDIS, which protected it from harm. He placed a sample of every extinct species within. Chronopolis looked different to every viewer. The Valeyard saw it as the Doctor's ancestral home (NOTDWU: He Jests at Scars...)