Stargate (Atari 2600)
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Stargate was the arcade sequel to Defender and is sometimes known as Defender II. The original arcade game was Stargate but most home ports were retitled Defender II because of copyright concerns. The Atari 2600 version was originally released as Stargate but a later release changed the name to Defender II. Other ports include the Atari 5200, Atari 8-bit computers, Atari 5200, NES, Apple II and DOS. The arcade version was also included in Midway Arcade Origins released in 2012 though it was also titled Defender II and not Stargate.
The game itself is similar to defender but with many more enemies and a stargate that serves different functions depending on the circumstances. For example, it may be used to transport your ship instantly to where a humanoid is in trouble or it may be used to skip levels.
Stargate/Defender II was a very good game though it never really came close to the popularity of Defender. The Atari 2600 port was also very good which was not always the case with arcade translations.