Mastocal was the ninth planet in the Sigma Kinna star system in the Klingon Empire in the Alpha or the Beta Quadrant. It was the only class M planet in its system, and it had two moons. It was the location of a major Klingon military base near the Klingon Neutral Zone (FASA RPG modules: The Klingons: Game Operations Manual, The Orions: Book of Common Knowledge).
The Mastocal base began construction in an area of space gained by the Klingon Empire during the Four Years War, close to the border with Federation space (FASA RPG module: Star Trek IV Sourcebook Update). It was completed in reference stardate 2/0109 (c. 2259), becoming the biggest Klingon naval base ever built at that time and marking the Empire's return to a state of war-readiness equal to what it had been before the Four Years War (FASA RPG modules: The Klingons: Star Fleet Intelligence Manual, The Klingons: Game Operations Manual, The Orions: Book of Common Knowledge). Starfleet Intelligence learned of its completion soon after (FASA RPG module: Star Trek IV Sourcebook Update).
From a base of operations at Mastocal, the Klingons planned to spread their influence and domination into Federation space. However, the Organian Peace Treaty put an end to these plans for war. Nevertheless, they continued building up Mastocal in preparation.
The Mastocal base was planet-wide and consisted of a number of facilities for training and R&R. There were also manufacturing plants for small weapons and tools. However, there were no ship construction yards, but extensive repairs could be made to any Klingon naval vessel in either the ground-based or orbital repair facilities.
It had a warm temperate climate, a thick class M atmosphere and a 21-hour-long day. It had a total surface area of 612,000,000 square kilometers and, with 31% land mass, a total land area of 189,720,000 square kilometers. About 33% of its makeup was normal metals, 10% was radioactive elements, 6% or trace deposits of industrial crystals, and it had only trace amounts of gemstones, and other special minerals (FASA RPG modules: The Klingons: Game Operations Manual, The Orions: Book of Common Knowledge).