Existing before the construction of the palace itself, the Bastion was a place of counsel and refuge for warlords and tyrants since before the Age of Strife. After the Unification Wars, it was absorbed into the growing Imperial Palace complex. During the Horus Heresy, the Bastion alongside the Phalanx became the command center of Rogal Dorn and the Imperial Fists, overseeing the defense of the entire Sol System.
- Grand Borealis Strategium - Dorn's central control room during the Siege of Terra.
- Sanctuary of Satya - The oldest part of the bastion and located on an off-shoot. This hall was open to the elements and held some of the last vegetation in the Inner Palace. During the fortification of Terra it was one of the places the Emperor demanded be left untouched alongside the Hall of Victories and the Great Chamber of the Senatorum Imperialis.