Summary
ONI's CASTLE Base is located on the planet Reach, 2,000 meters below the Highland Mountains. Protected by the massive amount of granite, it is virtually bombproof, well defended and impenetrable. CASTLE Base could withstand an 80 megaton nuke without a problem.
The base was destoryed by the NOVA in 2552 after Admiral Whitcomb ordered the planet destoryed after it's capture.