The Panopticon was the main room of the Capitol on Gallifrey. The Eye of Harmony was secretly kept under the Panopticon. (DW: The Deadly Assassin)
The Panopticon had six sides, as that number was revered on Gallifrey because it was the number of founders of Time Lord society. 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 Chapter House (Prydonian Chapter, Arcalian Chapter, Patrex Chapter, Dromeian Chapter, Cerulean Chapter and the Scendles Chapter) were involved in a competition to build a bigger statue of their founder than the other Chapter Houses, which resulted in the Scendles Chapter being bankrupt. (EDA: The Ancestor Cell) When Gallifrey's suns shined on the Panopticon, the interior would glow turqouise. (EDA: The Tomorrow Windows) The celling was so high that clouds formed within it and, upon occasion, it even rained. (EDA: The Eight Doctors)
The name panopticon was likely derived from opticon, the Gallifreyan word for the ancient village meeting place. (DWAN: Blind Fury)
Also in ancient times, the Pythia used panoptics placed throughout the city to watch over it. (NA: Cat's Cradle: Time's Crucible)