The Vault (The Pilot)

A Quantum Fold Chamber, also known as the Vault, containing, was hidden beneath the campus of St Luke's University in Bristol which the Twelfth Doctor made a promise to protect a really long time ago.

History
A long time ago on a planet, the Doctor was given the duty of executing the Time Lady known as Missy, by use of a technology able to kill a Time Lord off completely, however, the Doctor fiddled with the machine and it didn't kill her. After letting her live, the Doctor had to take care of her for 1000 years, after declaring an oath. (TV: Extremis)

By the 2010s, the Doctor and Nardole had spent almost seventy years guarding a vault on Earth that became the prison for Missy, while also working at St Luke's University, the premises beneath which the vault was located.

The Doctor and Bill Potts were chased to the location of the vault by a puddle of sentient oil which had taken on the image of a former student. At this time, both the Doctor and Nardole only left their posts when they realised that the threat was after them and showed no interest in the contents of the vault. (TV: The Pilot)

After being told by the Doctor to "leave [him] alone", a frustrated Nardole voiced his frustrations to himself while standing guard at the chamber before being interrupted as Missy started heavily knocking the door. Though acknowledging that the Doctor had been "distracted" by Bill, Nardole, hiding his fear, asserted to Missy that he would still stand guard and would not let it escape. (TV: Thin Ice)

After saving Bill and her friends from a creaking house filled with Dryads controlled by a mysterious landlord, the Doctor returned to the chamber with food to take his shift. He found Nardole checking the security of the chamber doors. Missy started playing Für Elise on a piano, gifted by the Doctor against the wishes of Nardole. Nardole was dismissed by the Doctor, who went on to tell Missy about his day. Hearing about people being eaten, the tune changed to Pop Goes The Weasel and the Doctor entered the chamber. (TV: Knock Knock)

The Doctor was standing guard in front of the chamber, waiting for Missy to come out. (TV: Extremis)