Defender was one of the most important titles of the Golden Age of Video Arcade Games, selling over 55,000 units to become the company's best selling game and one of the highest-grossing arcade games ever. Praise among critics focused on the game's audio-visuals and gameplay. It is frequently listed as one of Jarvis' best contributions to the video game industry, as well as one of the most difficult video games. Though not the first game to scroll horizontally, it created the genre of purely horizontal scrolling shooters. It inspired the development of other games and was followed by sequels and many imitations.
There were many ports to contemporary systems, most of them by either Atari, Inc. or its software label for non-Atari platforms, Atarisoft.' - Wikipedia