Source: http://www.megalextoria.com/wordpress/index.php/2017/06/01/stargate-atari-2600/
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.
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.
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