Halo Alpha
Halo Alpha

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CASTLE Base is an ONI base located on the UEG colony of Reach, inside Menachite Mountain, two thousand meters below the Highland Mountains. Protected by massive amounts of granite, reinforced concrete, plates of Titanium-A, and EMP-hardened metal,[1] it is virtually bombproof, well defended and virtually impenetrable.[2] CASTLE Base could withstand the force equivalent to an 80 megaton nuclear blast.

Dr. Catherine Halsey had an office in CASTLE Base, as did Colonel James Ackerson. The Spartan-IIs were familiar with CASTLE Base.


The complex was originally a titanium mine, but was closed down after the mine breached the Forerunner complex below.

During the Fall of Reach, the base was Delta Team's fallback position. William-043, Vinh-030 and Isaac-039 arrived here. They were soon accompanied by Frederic-104 and Kelly-087. It turns out that Dr. Halsey, accompanied by Jun-A266, was brought to the base as well in order to activate Operation: WHITE GLOVE to prevent the Covenant from getting UNSC data.

When the Covenant forces breached the base, Dr. Halsey was forced to put "Operation: WHITE GLOVE" into action, destroying the base and her assistant AI Kalmiya and preventing the Covenant from gaining any intelligence from the UNSC. Halsey and the SPARTAN-IIs retreated into the dangerous titanium mines below.

Known Sections[]

  • Level Aqua
    • Section Lambda
      • Armory
  • Level Scarlet
    • Omega Wing
      • Dr. Halsey's Office[3]
    • Medical Wing
      • Cold Storage Facilities
      • Surgical Bays
  • Level Lavender
    • Section Sigma
      • Elevator Shaft[4]



  1. Halo: First Strike, page 100, Although he barely felt the descent, Wagner knew he had dropped three kilometers into the planet, through solid layers of granite, reinforced concrete, plates of Titanium-A, and EMP-hardened metal. None of this made him feel any safer, though; ONI's research facility on Reach had the same setup, and it hadn't done those poor bastards any good.
  2. Halo: The Fall of Reach, page 229, Castle was a massive complex, two thousand meters below the granite protection of the Highland Mountains—bombproof, well defended, and impenetrable.
  3. Halo: The Fall of Reach, page 229, The private office was located deep underground, inside the "Omega Wing" of the super-secure ONI facility, code-named simply CASTLE.
  4. Halo: First Strike, page 134, "Down the elevator shaft in Section Sigma,"