The Emperor's Citadel, alternatively known as the Imperial Citadel, functioned as the clandestine fortress of the Galactic Empire. Situated on Byss, a planet nestled within the Deep Core, it served as the secluded throne world for Emperor Palpatine. A towering structure, distinguished by its black and red hues, stood prominently within the Imperial control sector, housing the Imperial throne room. It was here, in 10 ABY, that the resurrected Emperor Palpatine established his dominion over the Dark Empire.
This imposing tower, characterized by its black and red coloration, accommodated Palpatine's personal living spaces and administrative offices, along with those of his key personnel. The Emperor's throne room, Clone Labs that housed his collection of clone bodies, dark side Sith alchemical laboratories, the Great Hall, a war room, and libraries containing dark side texts were all components of the Emperor's Citadel.
Within this citadel, located on his throne world, Palpatine conducted experiments on a diverse array of life forms, including sentient beings, utilizing his dark side abilities. Subterranean breeding facilities were also present, responsible for the creation of the lethal Chrysalide rancors. The Imperial Citadel was defended by elite Imperial Sovereign Protectors, representing the most skilled members of the Imperial Royal Guard. He maintained his cloning operations within the palace to ensure swift and convenient resurrection whenever necessary.
The Palace compound also featured a series of training chambers where Palpatine instructed his Dark Side Adepts and a select group of Inquisitors. Furthermore, he possessed numerous Jedi and Sith holocrons that he had acquired. The Emperor's throne room could also function as a military command hub, which he utilized to oversee the World Devastator assault on Mon Calamari.
Construction of the citadel commenced shortly after Palpatine's ascension to power, following the conclusion of the Clone Wars. The Empire employed slaves from numerous conquered planets, including Gamorr, Toydaria, and Utapau, to expedite the citadel's completion.
After being reborn in a clone body following his demise aboard the second Death Star, Palpatine transported Luke Skywalker to the citadel, where he successfully converted the Jedi Master to the dark side. Subsequently, Leia Organa and Han Solo penetrated the citadel's defenses in order to rescue Luke. During the Battle of the Emperor's Citadel, New Republic operatives launched an assault on the citadel using sabotaged X-1 Vipers, but the attack was repelled through the use of alchemically-altered Chrysalides. The citadel, along with the rest of Byss, was obliterated in 11 ABY when the Eclipse II collided with the Galaxy Gun.
Three stormtrooper legions and 600 Imperial Sentinels served as the Citadel's garrison. Dark Side Adepts, Hunter-Killer probots, Scorpenek annihilator droids and Chrysalides also patrolled the grounds. Its equipment included HoloNet stations, battle computers and command rooms, the Clone Labs, prisons, a throne room, the Great Hall, a meeting area for the Imperial Inner Circle, libraries, museums, gardens and gladiator combat arenas.
In the rough draft of the script for A New Hope, the Imperial City was originally envisioned as being situated atop an incredibly tall spire, located far above the surface of the capital world. This concept would later inspire Cloud City in The Empire Strikes Back, and it may have also influenced the design of the Citadel on Byss.