The Capitol (also known as the Citadel) was the home of the Time Lords on Gallifrey. (TV: The Deadly Assassin)
The Capitol was protected by a large transparent dome. The Panopticon was the main room of the Capitol, where the Eye of Harmony was secretly kept far below.(TV: The Deadly Assassin)
The High Council met in the Capitol.There was a transmat system that linked the Council's room to the Death Zone. (TV: The Five Doctors) Also on Gallifrey was the Time Lord Academy, presumably in the Capitol. (TV:The Sound of Drums)
Underneath the Capitol, there were ducts and serviceways dating from the Old Time. (TV: The Deadly Assassin) By the time of the Doctor, these were used by those who worshipped shrines dedicated to the cult of "Rassilon the Vampire". (PROSE: Goth Opera)
In another area under the Capitol, at least two individuals had cameras pointed at a repair shop where TARDISes were being kept. (TV: The Name of the Doctor)
During the Last Great Time War the Citadel was severely damaged by Dalek Saucers. The crashed saucers remained on the surface next to the citadel. (TV: The End of Time)