The Citadel was a Forerunner superliminal communicator. Located on the edge of the core of Installation 00, the Citadel was the structure where one could remotely activate all of the Halo Array. It was defended by multiple shield-projecting Barrier Towers that had to be disengaged before it could be accessed.
Battle of Installation 00
During the Battle of Installation 00 in late 2552, the Prophet of Truth landed upon Installation 00 on the Forerunner Ship, and seized the Citadel while the majority of his fleet and armies distracted the pursuing United Nations Space Command/Swords of Sanghelios allied forces. However, Shipmaster Rtas 'Vadum "laid waste" to the Prophet of Truth's aerial fleet while John-117 and the Arbiter led a Marine and Sangheili ground attack on the Ark's surface, pushing forth to the Brute-held Citadel. The Prophet of Truth then activated the Barrier Towers, forcing the UNSC and the Elites to send strike teams to lower the Barrier Towers before gaining access to the Citadel. During the assault was Sergeant Major Avery J. Johnson captured by the Brutes, and he was taken to the Citadel, because the Prophet of Truth needed a human Reclaimer in order to activate the Halo array.
After the Barrier Towers were deactivated, the Assault Carrier Shadow of Intent moved over the Citadel and attempted to destroy it. This was interrupted by the Flood-controlled High Charity suddenly appearing out of slipspace and raining debris on the area, severely damaging the Shadow of Intent. The UNSC then launched an assault on the Citadel, led by John-117, with one M808B Scorpion Tank, multiple M12 Warthog LAAVs, one Mongoose, and multiple Hornet VTOLs, and a Pelican Dropship. The Covenant Loyalists defending the Citadel, landed a pair of Scarabs by an atmospheric landing, and supplemented it with one Wraith tank and a handful of Ghost and Prowler attack vehicles. Despite heavy resistance, John-117 and Thel 'Vadam were able to defeat the Loyalist defense, and with 343 Guilty Spark's assistance, enter the Citadel.
Meanwhile in the Ring Room of the Citadel, Truth was readying to activate the Halos, and a Brute Chieftain torturing the captive Johnson. When SPARTAN-117 radioed that he was too far from Truth to prevent him from activating the Halos, Commander Miranda Keyes boldly took a Pelican Dropship and crashed it into the Citadel, landing in the midst of Truth, his attendant Brutes, and Johnson. Keyes opened fire on the Brutes, killing one and injuring another before she realized that the situation was hopeless: she could not engage dozens of Brutes, then kill Truth, and then rescue Johnson. She aimed her Magnum at Johnson, offering to kill him and then herself, thus preventing the Covenant from firing the Halos, until Truth rose and shot her several times in the back with a Spiker, killing Miranda Keyes. Truth then forced Johnson's hand onto the control panel, activating the Halos.
- “How could I have known the Parasite would follow? Undoubtedly, this is the Heretic's doing!”
Meanwhile, the Flood were readying their forces against Truth: If the UNSC and Elites could not prevent Truth from firing the Halos, then the Flood would, and they began to scale the "outer reaches of the Citadel", joining forces with SPARTAN-117 and the Arbiter in a desperate attempt to deactivate the Halos. While Truth continued to sermonize, John-117, the Arbiter, and a Flood swarm attacked the chieftain and the Covenant Loyalists defending Truth, clearing the way for them to reach Truth. Upon reaching Truth, John-117 and the Arbiter find Johnson closing the eyelids of a now deceased Miranda Keyes. John-117 then quickly deactivated the Halos before they could complete their firing sequences, to Truth's dismay. The Arbiter then killed the defenseless Prophet of Truth with his Energy Sword.
As the Arbiter observed Truth's crumpled corpse, Johnson took Keyes' body inside her crashed Pelican. The Flood, however, betrayed both Master Chief and the Arbiter: Now that they had prevented the Halos from being activated, the Gravemind declared that it would lay waste to Master Chief and the Arbiter. As Master Chief and the Arbiter tried to board the Pelican, the Flood were able to knock John and the Arbiter to the ground and nearly causing Johnson to crash. As a result, the two had to battle Flood on their way out.
Having escaped the Flood, John-117 saw the cryptic figure of Cortana, and he tentatively followed the flickering figure to an open balcony where he activated a panel, shortly after which a massive ring rose from the center of Installation 00. 343 Guilty Spark announced that this was another Halo installation, constructed by the Ark's foundries to replace the destroyed Installation 04 and asked Master Chief what he would do. Master Chief replied that he would activate it, and 343 Guilty Spark hurried off excitedly to make the final preparations. The Arbiter asked John-117 exactly how he planned to do such a thing, due to the fact that he will need an Activation Index in order to activate Halo. John-117 then looked at the crashed hulk of Flood-infested High Charity nearby, where the real Cortana is trapped, knowing full well that Cortana has possession of the Index from the first Halo ring.
- If you make it to the left side of the Citadel on a high, perched corner you will hear the song Siege of Madrigal. If you make it to the right side, you will find the Microsoft Sam Easter egg.