Prime Base is the tremendous military base that first grew from "The Hill", a man-made, flat-topped, steel sheathed mountain that was the main base of first, the Triplanetary Service, and then of the new Galactic Patrol on Tellus.
The three (plus new members Loren Wiseman and John Harshman) began drafting a variety of designs, some derivative of existing games at the time (with generic names like Guerre, Swamp), and some original concepts (Triplanetary).