According to the Minister of Enlightenment, a cloister was a "covered walkway", "an architectural feature of ecclesiastical and educational establishments on Earth". (TV: Four to Doomsday) The Doctor's TARDIS contained a room the architecture of which mimicked that of a cloister. (TV: Logopolis)
Underneath the Capitol on Gallifrey, was the Cloisters, which housed an entrance to the Matrix. The Cloisters were inhabited by Wraiths, colloquially named as Sliders, who protected the entrance, acting somewhat as a firewall, according to the Twelfth Doctor. As a student in the Academy, a young Doctor had wandered into the Cloisters and, with the help of the Sliders, had managed to find a way out.
Many creatures had tried to invade the Cloisters of Gallifrey in what was known as the Cloister Wars, including the Daleks, Cybermen, and Weeping Angels. The Twelfth Doctor returned there after the Time War had ended. (TV: Hell Bent)