Originally a balcony and eventually Archive Chamber 403, one room in the Citadel had many uses over Gallifreyan history.
At the time of Omega's sacrifice, the space was an open balcony overlooking the Old Harbour. It was at the highest level of the Citadel at the time. The other and Patience spoke at the balcony and watched the Gallifreyan fleet in the starharbour below before they left to Qqaba.
Over the next few millions of years, as the Capitol changed and grew, the balcony became an indoor room which kept the space's original shape. Various masonry indicated a deep history of renovation. The room had hundreds of uses over time, including being an Ordinal's office and student lodgings for the Time Lord Academy.
At the time of the Infinity Doctor, the area of the Citadel had become the Archives, with the room being the rarely used "Chamber 403" with numerous display cases and cabinets containing long-forgotten things. It contained a giant three-winged stuffed bird in a suspensor field, a pile of oily cogwheels from a discarded giant clock mechanism, a battered wardrobe, Tegorak's armour, and Savar's TARDIS key. Under Omega's control, Savar stole the key to open the Needle, with the theft being investigated by Raimor and Peltroc. (PROSE: The Infinity Doctors)
Behind the scenes
Appearing early in the novel, the contents of "Chamber 403" establish The Infinity Doctors's tone by rapid-fire referencing various obscure Doctor Who fanfictions, most of which have not yet been identified on this wiki.