Stargate is the sequel to the 1980 Arcade game Defender, released later the same year. The gameplay is functionally identical to its predecessor, maintaining the horizontal wraparound stages and the goal of eliminating the enemy landers before they can kidnap the humans on the ground. The game adds new enemies, new tools like a limited shield and various "stargate" portals that send players across the stage and bonus stages every five stages.
The game was released as Stargate for its Arcade debut, but was renamed Defender II for most of its home versions, with the two names becoming interchangeable. The Atari 2600 version was first released as Stargate and rebranded as Defender II after its re-release. The game was named Stargate (or Star Gate) for its Japanese Famicom release and Defender II for its North American NES release.
The game has no connection to the Stargate movie and TV show franchise.