Installation 00 Citadel

The Citadel was a Forerunner superluminal communicator building.

Introduction
It was located on the edge of the core of Installation 00, and was a location from which one could remotely activate all of the Halo Installations. The Citadel was defended by multiple shield-projecting Barrier Towers that had to be disengaged before it could be accessed. The citadel is where the prophet of truth activated the rings

Beginning
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/Covenant Separatists allied forces. However, Ship Master Rtas 'Vadum "laid waste" to the Prophet of Truth's aerial fleet and SPARTAN-117 and the Arbiter led a Marine and Sangheili ground attack on the Ark's surface, pushing forth to the Brute-held Citadel. However, the Prophet of Truth 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, however, Sergeant Major Avery J. Johnson was captured by the Brutes, and was taken to the Citadel, where the Prophet of Truth was sermonizing before activating the Halo array.

UNSC Assault
After the Barrier Towers were deactivated, the UNSC launched an assault on the Citadel, with one M808B Scorpion Tank, one M12 Warthog LAAV, one Mongoose, and multiple Hornet VTOLs, and a Pelican Dropship. However, the Covenant Loyalists defended the Citadel, landing 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, SPARTAN-117 and the Arbiter were able to defeat the Loyalist defense, and with 343 Guilty Spark's assistance, enter the Citadel.

However, in the Ring Room of the Citadel, Truth was readying to activate the Halos, and one of his Brute Chieftains was 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 several 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 in a coup de grace, until Truth rose and shot her several times in the back with a Spiker, killing Keyes.

The Flood
"How could I have known the Parasite would follow? Undoubtedly, this is the Heretic's doing!"

- Prophet of Truth

Meanwhile, the Flood were readying their forces against Truth: if the UNSC could not prevent Truth from firing the Halos, the Flood would, and they began to scale the "outer reaches of the Citadel", offering their aid to SPARTAN-117 and the Arbiter in killing Truth. While Truth continued to sermonize and procrastinate, SPARTAN-117, the Arbiter, and a Flood swarm attacked the Brute Chieftains, Jumpack Brutes, and Grunts defending Truth, sweeping them aside as the Arbiter took an Energy Sword to Truth, stabbing the Prophet of Truth and killing him before he could activate the Halos.

As the Arbiter observed Truth's crumpled corpse, Johnson shut Keyes's eyelids, taking her body inside her crashed Pelican. The Flood, however, changed allegiances: now that they had prevented Halo from being activated, the Gravemind declared that it would lay waste to SPARTAN-117 and the Arbiter. As Johnson fled in Keyes's former Pelican, SPARTAN-117 and the Arbiter battled their way out of the Citadel, escaping to the lower reaches of the tower, fleeing from the Flood.

There, SPARTAN-117 saw the cryptic figure of Cortana, and he tentatively followed the flickering figure to an open balcony where a massive ring rose from the ground 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 if SPARTAN-117 would light the ring and activate a localized tactical pulse to cleanse the Flood from the Ark. The Spartan announced that he would, and 343 Guilty Spark hurried off to make the final preparations when the Master Chief and the Arbiter noted the crashed hulk of Flood-infested High Charity nearby. SPARTAN-117 declared that he would enter High Charity and save Cortana as the Arbiter left to coordinate a joint UNSC/Covenant Separatist retreat as SPARTAN-117 would later activate the reborn Halo installation.