Castrovalva was a city created by the Master, located on a unnamed planet. The Master, using projections created by Adric, created a false databank in the freshly regenerated fifth Doctor's TARDIS. When Nyssa was looking for something to smooth the Doctor's latest regeneration she found an entry for Castrovalva.
The people of the city appear to be very primitive, but peaceful. There was a library in the city which the Doctor visited, and read up on the history of Castrovalva, which was also false. This gave the Doctor a suspicion that things were not correct.
The whole of Castrovalva was a huge block transfer computation, with Adric providing much of the mathematics. The Master had trapped Adric inside a web, although the Doctor did manage to free him. In doing so, he doomed Castrovalva. The Doctor, Tegan, Nyssa and Adric escaped. The Master did also.
- Small planet of the Phylox Series, Andromeda