The Imperial Palace is the seat of the Adeptus Terra and the center of imperial power and administrative authority on Terra. It spreads across the whole of what used to be the Himalayan Mountains of Old Earth and is primarily defended by the Adeptus Custodes, although Space Marine, Collegia Titanica, and Adepta Sororitas forces are also known to protect the massive complex. The Imperial Palace is not just a single building but a massive interconnected series of continent-wide fortifications comprised of cyclopean and pyramidal structures that extend for several kilometres into the sky and for several dozen kilometres beneath the surface of Terra. The Palace is said to have 4,029,854 chambers in its entirety. This complex is divided into two security perimeters which essentially function as separate urban regions, the Outer and Inner Palaces, both of which are marked by thousands of landing pads for small craft and monstrous defensive weapons systems. Many of these defences were designed and emplaced by the Primarch Rogal Dorn at the time of the Horus Heresy. The Imperial Palace still possess the scars of the terrible conflict that raged around it during the Battle of Terra more than 10,000 standard years ago. The heart of the Inner Palace contains the Sanctum Imperialis, the Hall of the Golden Throne, deep within the Inner Palace past the Eternity Gate where the Emperor of Mankind resides, his dying and decaying body interred within the arcane mechanism of the Golden Throne in stasis. Only the members of the Adeptus Custodes and a few select Imperial servants are allowed to pass through the Inner Palace's Eternity Gate and enter the presence of the Emperor of Mankind. Although billions of people work in the Palace complex, very few people have ever made it inside the Hall of the Golden Throne, one of that select number being the Inquisitor Jaq Draco.

The Imperial Palace is as heavily populated and just as active as any hive city of the Imperium. In fact, it probably has a higher density of population per square mile than most hive cities. Billions of Adepts from all branches of the Adeptus Terra work in the Imperial Palace complex overseeing the affairs of Mankind. The Outer Palace has become a grim slum inhabited by millions of destitute people who have no employment beyond the few scraps they can squeeze from the tables of the more fortunate Adepts. While the Adeptus Custodes is responsible for the overall security of the Imperial Palace and directly polices the affairs of the Inner Palace, security within the urban zone of the Outer Palace is normally provided by the Adeptus Arbites. The Arbitrators are particularly concerned with providing security outside the many Imperial governmental office buildings of the Outer Palace where thousands of people queue up every day to seek employment in the Adeptus Administratum, even if only as menial serfs. Such employment is better than the desperate fight for survival that confronts so many of the men and women who make their homes in the squalid back alleys of the Outer Palace. Arbites shock troops are always on hand to brutally suppress the inevitable queue riots that break out every day as the grim lines of the desperate poor jockey for position.

The Imperial Palace is the very heart of the Administratum, whose countless bureaucratic Adepts labour anonymously in its massive archives, some stretching many kilometres beneath the surface, to keep the galaxy-spanning empire of Mankind running smoothly in the name of the God-Emperor. Without these Adepts, the administrative machinery of the Imperium would grind to a halt, all access to the records of the past and the lines of transmission for decisions about the future cut off from the million worlds of humanity. The Inner Palace is also the seat of the Senatorum Imperialis, the twelve High Lords of Terra, who are charged with divining the Emperor's will and ruling the Imperium in his name, their decisions implemented by the Adepts of the Administratum and all the other organs of Imperial government. These twelve men and women, often drawn from the highest nobility of the human-ruled galaxy, are the single most powerful individuals in the Imperium. They ultimately concern themselves with the implementation of humanity's grand strategy, guiding the deployment of the Emperor's armies and ensuring that his will is respected on the million planets of the Milky Way Galaxy that Mankind calls home.

The Sanctum Imperialis

The Inner Palace contains the headquarters of the most important Imperial institutions, such as the Astronomican, the Senatorum Imperialis and the Sanctum Imperialis, the Throne Room of the Emperor of Mankind himself. The entrance to the Sanctum Imperialis is protected by the Eternity Gate. Two Battle Titans of the Legio Ignatum stand guard at each side of the massive portal, sharing their eternal vigil with the 10,000 members of the Adeptus Custodes. The grand hallway of the Inner Palace that leads to the Eternity Gate, called the Hall of Heroes, is filled with the banners of every Imperial Guard Regiment and Space Marine Chapter that has ever loyally served the Emperor. At the end of the Hall of Heroes lies the Eternity Gate itself, forged of adamantium that has been heavily layered with ceramite to provide extra protection from weapons fire. The outer layer of the gate is covered in beaten gold that has been sculpted by some of the greatest artists of the Imperium into an heroic icon of the Emperor in the days of the Great Crusade, standing triumphant over all the foes of Mankind. An elite cadre of the Adeptus Custodes called the Companions, 300 of the most skilled and highly trained warriors in the Imperium, guard the Sanctum Imperialis while their brethren are responsible for the overall security of the entire Palace complex. During the Battle of Terra, the Forces of Chaos managed to penetrate the defences of the Outer Palace and make their way to the Eternity Gate, which was successfully defended by the Primarch Sanguinius while the Emperor remained seated on the Golden Throne within. Following the successful defence of the Gate, the Emperor would teleport to the battle barge Vengeful Spirit of the Warmaster Horus in orbit of Terra to bring the Horus Heresy to its climax.

Deep within the Sanctum Imperialis, protected at all costs by the Adeptus Custodes, lies the arcane mechanism of the Golden Throne, within which is interred the decaying form of the Emperor. The Master of Mankind's shattered body is kept barely alive by the life support mechanisms of the Throne so that his mind might have the necessary anchor in the physical realm so that it can remain active in the Warp, where for 10,000 years the Emperor has guided the psychic beacon of the Astronomican and defended Mankind from the machinations of Chaos. Hordes of Tech-priests from the Adeptus Mechanicus scurry about the Sanctum Imperialis, performing the myriad tasks that they do not understand but are required to keep the Golden Throne's ancient mechanisms functioning. In a large arena constructed in front of the Golden Throne psykers brought by the Imperium's infamous Black Ships from across the galaxy are soul-bound to the Emperor so that they may enter the service of his Imperium as Sanctioned Psykers.

Sources

• Warhammer 40,000 Rulebook (4th Edition)
• Warhammer 40,000 Rulebook (5th Edition)
• Inquisition War Omnibus (Novel) by Ian Watson