Room 12

Room 12 was a room within the Twelfth Doctor's confession dial that contained a portal to Gallifrey, behind a azbantium wall. The Doctor, as part of a four-and-a-half-billion year (TV: Heaven Sent, Hell Bent) cycle, wore down the wall until he was able to escape. Unlike the other rooms in the confession dial, it did not reset when the Doctor left it. (TV: Heaven Sent)