Defender

Defender is a 1981 horizontally scrolling shooter video game developed by Williams Electronics for arcades. The game is set on either an unnamed planet or city (depending on platform) where the player must defeat waves of invading aliens while protecting astronauts. Development was led by Eugene Jarvis, a pinball programmer at Williams; Defender was Jarvis's first video game project and drew inspiration from Space Invaders and Asteroids. Defender was demonstrated in late 1980 and was released in March 1981. It was distributed in Japan by Taito.

Defender was one of the most important titles of the golden age of arcade video 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's best contributions to the video game industry and one of the most difficult video games. Though not the first game to scroll horizontally, it created the genre of horizontal scrolling shoot 'em ups. It inspired the development of other games and was followed by sequels and many imitations.

Ports were developed for contemporary game systems, most of them by either Atari, Inc. or its software label for non-Atari platforms, Atarisoft. The 1982 Atari 2600 version was one of the best-selling games for the system and sold over 3 million cartridges.

source: Wikipedia