Starbase Yorktown was a massive Federation space station located on the frontier, near the Necro Cloud nebula, in the alternate reality. Containing millions of individuals, it was Starfleet's newest and most advanced base as of 2263 and was commanded by Commodore Paris.
Yorktown was constructed in lieu of a planetside settlement so as not to show favoritism to any particular recently-admitted worlds.
Three years into his five-year mission aboard the USS Enterprise, Captain James T. Kirk applied for a vice admiral position at Yorktown. At the time, Starfleet's most technologically-advanced ship was under construction there.
Attack by KrallEdit
In 2263, the Enterprise docked in one of Yorktown's space lanes for resupply. From there it was dispatched to the Necro Cloud on a mission to ostensibly rescue the stranded crew of Kalara. In truth, the Enterprise had been lured into the nebula by Krall where it was attacked and destroyed at the planet Altamid.
Marooned on Altamid, the surviving crew of the Enterprise discovered that Krall had used a Magellan probe to retrieve data from the Federation including schematics of Yorktown, which he meant to attack. Launching a fleet of Swarm ships, Krall attempted to deploy the super weapon known as the Abronath in Yorktown's life support system in order to wipe out all life on the base and use to it to strike other Federation worlds.
Piloting the 22nd century era USS Franklin into Yorktown in pursuit of Krall, Captain Kirk crashed the old Starfleet ship at Yorktown Central Plaza, then engaged with Krall, ultimately killing the despot before he could deploy the Abronath.
In honor of his heroics, Kirk was offered the vice admiral position at Yorktown, but declined in favor of continuing to command a starship. (Star Trek Beyond)
Yorktown's structure consisted of a matrix of city-sized interlocking rings and radiating arms enclosed in a spherical translucent surface; Enterprise doctor Leonard McCoy likened it to a giant "snow globe" in space. Several of the station's arms reached the surface of the sphere and mounted doors large enough to admit starships into internal space lanes.
The arms of the starbase met at a central nexus point where a central plaza and Yorktown Headquarters stood. Due to the complex gravitational characteristics of the structure, the center of Yorktown contained a gravitational slipstream.
Yorktown contained a shipyard and was defended by numerous turrets on its surface as well as a network of satellites, all armed with phasers and photon torpedoes. The station's military and manufacturing capabilities were sufficient for it to pose a major threat to the Federation if captured intact by an enemy. It housed several million beings from across the Federation, including the family of Hikaru Sulu. (Star Trek Beyond)
Starbase Yorktown was designed by Sean Hargreaves. According to Hargreaves, the initial design of Yorktown's arms did not feature windows looking into the passageway, however this feature was added in order to depict the Enterprise and later the Franklin flying through interior structure of the base. This addition also led to the sequence in which the Franklin rises up through a reflecting pool at Yorktown Central Plaza. The buildings in Yorktown were designed to match the real life buildings at the filming location in Dubai, United Arab Emirates. Also according to Sean Hargreaves, each of the arms of Yorktown was intended to be 17½ miles long. 
The digital model of Yorktown was constructed by Hargreaves, a team of four modelers and two texturers at Double Negative studios in Vancouver, British Columbia using 810 unique modular elements detailing 64 cityscapes. The sequence first featuring Yorktown in Star Trek Beyond consisted of 1,000 frames, depicting the 1.3 trillion polygon digital model of the base. Digital city arms, shell background, CG crowds and air traffic were added by Double Negative to footage captured during a monthlong location shoot in Dubai. (Cinefex, No. 148)