At the start of the episode, the Doctor and Romana are in peril because the TARDIS "defense shield" is no longer functioning. However, it has been made clear in previous episodes (including The Curse of Peladon: Episode One (1972)) that the TARDIS is indestructible. So there should be no need for it to even have a defense shield.
At the start of this story, the Tardis console is disabled due to the Doctor's tinkering. When the emergency occurs the Tardis cannot escape, however it is established that the Tardis has an alternative control room with wood panelled walls and console. The Doctor could easily have switched control of the Tardis to this control room.