From Halopedia, the Halo wiki
The base was created long ago to fight "earlier and simpler" enemies than those faced by the UNSC in the 26th century, and was at one point occupied by NATO in the past. It was recommissioned by the UNSC following the New Mombasa Slipspace Event. Following their rise to prominence in the hands of Jiralhanae soldiers in the Great Schism, recovered examples of the Type-2 spike grenade and the Type-25 Spiker were tested at the facility.
Outpost C9 consists of a base structure built into a small canyon, with the only point of access being the open water at the canyon's fore. To prevent coastal assaults, the primary defense of the base involves a large solid wall, watchtower and a bunker watching down over the beach. The middle of the wall contains a large blast gate which can be opened by a computer terminal inside the base, and a single AIE-486H machine gun is situated over the gate to deter potential intruders. The interior of the base contains a command center, hangar and a courtyard at the back, among other buildings. The courtyard contains a static surface-to-air missile launcher for firing ASRGAM 10X missiles, and the hangar are has several additional locked doors that lead further into the base.
Due to the base's age and abandonment followed by a swift UNSC recommissioning, many of the structures in the base have crumbled over time, but are mixed with more modern and up-to-date technology of the 26th Century.
List of appearance