Caerdroia

You may be looking for the audio story of the same name.

Caerdroia was a crucial place in the Divergent Universe. Its name was a Welsh word that meant, according to the Eighth Doctor, "the fortress of many turnings... a maze." It had a castle, as the Kro'ka said, everywhere. This castle was, the Doctor surmised, the "home office" of the Divergence. Ultimately, Caerdroia was where the Doctor was reunited with his TARDIS after an extended time living in the Divergent Universe without it. (AUDIO: Caerdroia)