The Panopticon was the main room of the Capitol on Gallifrey. It served as the Time Lords' parliament and seat of State. (PROSE: Engines of War) The Eye of Harmony was secretly kept under the Panopticon. (TV: The Deadly Assassin)
The Panopticon had six sides, one for each of the Founders of Gallifrey. At every corner of the Panopticon, there were six black statues of the six founders of Time Lord society (Rassilon, Omega, Pandak, the Other, Apeiron and one other, possibly Eutenoyar). Every college (Prydonian Chapter, Arcalian Chapter, Patrex Chapter, Dromeian Chapter, Cerulean Chapter and the Scendeles Chapter) was involved in a competition to build a bigger statue of their founder than the other Chapterhouses, which resulted in the Scendeles Chapter being bankrupted. (PROSE: The Ancestor Cell) When Gallifrey's suns shone on the Panopticon, the interior glowed turquoise. (PROSE: The Tomorrow Windows) The Panopticon's ceiling was so high that clouds formed near it, and sometimes it rained. (PROSE: The Eight Doctors, Warmonger)
In ancient times, the Pythia used panoptics placed throughout the city to watch over it. (PROSE: Cat's Cradle: Time's Crucible)
As a child, the First Doctor played in the tunnels under the Panopticon. (AUDIO: Order of the Daleks)
When Rassilon allied himself with the Cybermen, he redecorated the Panopticon. He took the Twelfth Doctor and the General there. (COMIC: Supremacy of the Cybermen)
After taking the Thirteenth Doctor to the ruins of Gallifrey, the Master points to the door leading to the remains of the Panopticon and reminisces about the times he and the Doctor had inside of it. (TV: The Timeless Children)
Behind the scenes
The name Panopticon (derived from Greek: "pan" meaning "all" and "Opticon" meaning "view") was coined by Jeremy Bentham for a revolutionary approach to the way prisons should be designed.