From Halopedia, the Halo wiki
In early 2558 this location was held by Promethean Crawlers and Promethean Watchers, and it was north of the UNSC's operation of influence. Fireteam Crimson was sent to the "Fortress" to deactivate a Forerunner tower by Commander Sarah Palmer. After taking down Forerunner communications jammers, Crimson cleared the area of Prometheans and deactivated the Forerunner structures and were extracted.
Hacksaw Squad was sent to hold the "Fortress," but came under heavy attack by Covenant forces. Crimson was sent in after Hacksaw radioed Palmer for assistance. Clearing through the Covenant, Robert Dalton asked Crimson and Hacksaw to go to the top of the Forerunner tower for evac. However, several Phantoms arrived carrying high-ranking Sangheili. They were defeated, and Crimson and what remained of Hacksaw's survivors, if any, were transported away in a Pelican.
Later, Covenant forces retook the position and Jul 'Mdama fled here from "Two Giants" after his location was discovered via Slipspace translocation. Crimson followed and arrived at the "Fortress," but was too late to catch 'Mdama as he had left to "The Cauldron" through another portal. Clearing the "Fortress" of Covenant, Crimson pursued 'Mdama, exiting the "Fortress" through a portal. 
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