Bobby Kotick resided and ruled for an unknown amount of time (presumably since his takeover of The Underworld 200o years ago) as well as that of his hundreds of Queens. Some time ago it became the headquarters of Activision. It is unknown whether it was built for this purpose, or if Kotick found it and took it as is.
After being found, kidnapped, and brainwashed by Bobby Kotick, Activision Castle served as the home of Nico while she was serving as Kotick's queen. The rest of the heroes stumbled upon the castle in the Ninth Level of Hell during their search for her, after a run-in with Chibi.
Here, the heroes fought and defeated Bobby Kotick, rescued Nico, and Hades mutinized against Kotick to take control. It is unknown whether or not it continues to serve as the home of Kotick or HQ of Activision after Smoosh's departure from Hell and the ascending of Hades into power.