The Sanctum of the Hierarchs is the private quarters of the High Prophets on High Charity. They are restricted to virtually all other creatures, excepting the Honor Guard and possibly Prophet and Elite Councillors.
The Inner Sanctum features unique architecture, highly ornamented doors, and various other luxuries. The Sanctum has several entrances; One is via a lift in the Council Chamber, another is a large hallway where dozens of Honor Guards stand watch. The higher levels of the sanctum or the Far Tower also include three tenders for Phantom dropships.
Many events took place here, including the "exchange of hats" when the Brutes replaced the Elites as the Honor Guard, after the assassination of the Prophet of Regret. It was here that the Prophet of Truth and the Prophet of Mercy explained to the arbiter his task of landing on the surface of Installation 05 and retrieving the Index from the Flood infested Library, which took place in two levels through the Sentinel Wall and the Quarantine Zone.
In the end, the Flood used Pelican dropships to land in the sanctum. It was one of the first areas to be infested; it was filled with spores and Flood "structures" when the Master Chief traversed the area in search of the High Prophet of Truth.