The Zemtar fortress was a building on Rigel VII.
It was traditionally used as the primary barracks of the Kaylar warrior caste; however, by 2254 it has been abandoned as the Kaylar were disbanded as a symbol of Rigel's rejection of its more primitive traditions. It was to be the site of Rigel VII's ratification ceremony in joining the United Federation of Planets. However, the ceremony never took place due to a Kaylar uprising on the planet. Whilst the uprising was taking place, Captain Christopher Pike of the USS Enterprise was visiting the fortress with his Rigellian guide Talza - who was actually involved in the Kaylar conspiracy and had led Pike into an ambush, the Captain barely escaping alive. (EV comic: "Our Dearest Blood")
The incident was used shortly after by the Talosians to create a mental illusion when they held Captain Pike in captivity. However, in this version of events Talza was replaced by Vina, who remained with Pike throughout the battle; in the real attack, Talza fled to continue her part in the conspiracy elsewhere. (TOS episode: "The Cage")