From Halopedia, the Halo wiki
The site designated "Lockup" is believed to have been used as a staging, repair and refit site for mid-size Forerunner ships. Data acquisition also hints that the site may have been involved in the first deployment of Forerunner ships in the Human-Forerunner War, specifically the Didact’s own ship, Mantle's Approach.
During the Requiem Campaign, Icebreaker squad was captured and sent here; after Fireteam Crimson was captured, they were sent here as well. Later, Roland located Crimson and Miller quickly ordered a diversionary bombardment from Dalton. After escaping and killing the guards, Crimson made their way through the facility attempting to find a way to escape. Miller decided Crimson must hijack a Phantom to escape as they were too deep behind enemy lines for friendly forces to rescue them. Covenant forces guarding the area included a nearby Zanar-pattern light cruiser. However, after entering the next area, Roland detected Icebreaker squad. Crimson engaged the guards, which included Promethean Crawlers. After freeing the Marines, one of them, Lieutenant TJ Murphy, said he could remotely take over a nearby Phantom. Crimson and the other Marines provided cover for him while he did so, holding off several waves of Covenant troops. Eventually, Lieutenant Murphy was able to take over the Phantom, allowing Crimson and Icebreaker squad to escape.
In exchange for his life, a captured Sangheili revealed the location of a Covenant listening post at "Lockup." Crimson and a squad of Marines were subsequently dispatched to raid "Lockup" and destroy the listening post. After deactivating several jammers, they found the Covenant were using a captured Army Pelican, shot down at "the Cauldron" and assumed destroyed, to listen in on UNSC communications. Crimson eliminated the Covenant forces in the area and deactivated the energy tethers tied into the Pelican's communications systems. Once Crimson was evacuated in another Pelican piloted by Murphy, Robert Dalton destroyed the Pelican with an orbital strike, preventing the Covenant from using it again and ending their spying on UNSC communications.
Crimson later returned to Lockup while searching for Dr. Catherine Halsey. They were dropped off with some Marines and secured the area. They moved on to the next area and eliminated the Covenant forces there as well. Dalton detected Marine IFF tags nearby, but when Crimson went to investigate they were ambushed by Covenant forces. Crimson was forced to hold off multiple waves of reinforcements including Wraiths and Hunter pairs. As they were about to be picked up, Dalton detected more Covenant reinforcements incoming. Crimson managed to hold off additional reinforcements, now including Promethean Crawlers. During the fight, Poker Squad called for assistance. When Crimson defeated the enemy forces in the area they moved to the next and assisted Poker Squad in eliminating the enemy forces.
While assisting Poker, Roland detected Hawk Squad's IFF tags, and once Poker Squad was safe, Crimson was sent to investigate. After fighting through numerous Promethean forces they arrived at Hawk's position to find only their IFF tags. Jared Miller accessed a recording on one of the tags that revealed they had been captured by Elites looking for Reclaimers. Hawk was killed while the Elites tried to activate a device. Crimson activated the device while holding off Elite reinforcements. It was soon revealed to be a map of Requiem, Roland quickly contacted Dr. Glassman and believed the map could help them free the UNSC Infinity from Requiem.
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