Chronotis's TARDIS

Professor Chronotis' TARDIS was disguised as the living quarters of Professor Chronotis at St. Cedd's College, Cambridge University. It was capable of holding another TARDIS within its force field and creating a bridge between the two to walk through.

This TARDIS was a Type 12, Mark 1 TARDIS, a much earlier version than the Doctor's Type 40, which Chronotis had salvaged from the scrap heap before leaving Gallifrey. It could interfere in an individual's time stream, as Clare Keightley discovered when she reactivated the ship's controls, causing the previously deceased Chronotis to come back to life. (WC: Shada, AUDIO: Shada, PROSE: Shada)

Behind the scenes
Chronotis' TARDIS was identified as a Type 12, Mark 1 in the novelisation of Shada. Its type wasn't identified in any other version of the story.