The One True City is Camelot, the central city founded by three Sword Brothers that pulled the broken world together and started a brief golden age.
The One True City
Initially imagined by Arthur, the building of the One True City pulled the three Realms together in peace and harmony, just as it pulled the land together. After many years of success, infighting began to tear the place apart until the Second Breaking of the world occurred, and Camelot was torn apart, along with the lands it had pulled together.
A significant part of the One True City's nature is that it was supposed to belong to all of the Realms but many could not understand that, leading to the city's downfall.