The Capitol (also known as the Citadel) was the home of the Time Lords on Gallifrey. (TV: The Deadly Assassin)
The Capitol contained the Panopticon and, buried far beneath it, the Eye of Harmony. 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)
The High Council met in the Capitol and there was a transmat system that linked the Council's room to the Death Zone. Other Time Lord buildings on Gallifrey included the Time Lord Academy. (TV: The Five Doctors, The Sound of Drums)
Underneath the Capitol, at least two guards had security cameras pointed at a repair shop where TARDISes are being kept. (TV: The Name of the Doctor)
During the Last Great Time War the Citadel was badly damaged by Dalek Saucers even though the Capitol was protected by a system of dual-turrets that shot down the saucers. The crashed saucers remained on the surface next to the citadel. (TV: The End of Time)