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It was hidden two kilometers directly underneath Menachite Mountain, part of the Highland Mountains in the Viery Territory.[Note 1] Protected by massive amounts of granite, reinforced concrete, plates of Titanium-A armor, and EMP-hardened metal, it was virtually bombproof, well defended, and nigh-impenetrable.
Dr. Catherine Halsey had an office in CASTLE Base's Omega Wing, as did Colonel James Ackerson. The SPARTAN-IIs were familiar with CASTLE Base, some of their training having taken place in the base as well as the abandoned mine tunnels around it.
The complex that would later become CASTLE base was originally a titanium mine, but was closed down in the 2520s or 2530s[Note 2] after the mine breached the Forerunner complex below. The mine would be used for storage for several years before being put into action by the Office of Naval Intelligence.
Fall of Reach
During the Fall of Reach, the base was Team Delta's fallback position. William-043, Vinh-030, and Isaac-039 arrived there. They were soon joined by Frederic-104 and Kelly-087. There, they discovered Dr. Halsey, who had remained behind to initiate Operation: WHITE GLOVE, destroying the base to prevent the Covenant from acquiring advanced UNSC technologies.
When the Covenant forces breached the base, Dr. Halsey was forced to put Operation: WHITE GLOVE into action, destroying the base and deleting her AI assistant Kalmiya. This prevented the Covenant from gaining any intelligence from the base. Halsey and the Spartans then retreated into the dangerous titanium mines below, and later to the Forerunner complex, where they discovered a powerful Forerunner crystal.
Return to CASTLE base
In October 2559, Blue Team once again returned to CASTLE base seven years following the base's destruction. Following the emergence of the Created led by Cortana and the subsequent conflict, Blue Team were ordered to breach the rubble of CASTLE and recover assets from Halsey's abandoned laboratory, believed to be key to defeating Cortana's forces. Despite the destruction of the base itself, the lab survived, guided by a subroutine that Kalmiya had left behind to protect the cryo-vault. With some difficulty, Blue Team succeeded in extracting the assets back to the UNSC Infinity.
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