Starbase 535 was a Federation space station, a variant K-class starbase in Starbase service in the 23rd century. In the 2270s decade, Starbase 535 was located near a gas giant at the Federation border. A separate drydock had been built inside a nearby asteroid. (STO - Agents of Yesterday mission: "Days of Doom")
In the year 2270, the station's commanding officer was a male Human. Commander Montgomery Scott was aboard the asteroid drydock when a hostile fleet attacked the Federation border system. A squadron of uptime Khitomer Alliance starships from the year 2410 arrived in the system, disguised as contemporary Federation vessels. The Allies defended the station and faced off against a doomsday machine the Na'kuhl had delivered to the hostile attackers. Scott led the drydock's engineers to produce warp cores, including one meant for a Atlas-class dreadnought, to stop the doomsday machine.
Slowing the doomsday machine by dropping Scotty's warp cores in its path and holding off the hostiles, provided the engineers with the time to ready the dreadnought's warp core. The Allies saved the starbase by halting the doomsday machine with the largest warp core, and the hostiles retreated from the Federation border, foiling the Na'kuhl's plan to upend the timeline. (STO - Agents of Yesterday mission: "Days of Doom")