Genre: Shoot'em up

A shoot ’em up is a game where your ship, sub or soldier faces wave after wave of enemies and the fire button rarely rests. On the Atari 2600 the genre had to cope with tight limits: the graphics chip can draw only two player sprites and two missiles on a line, so more enemies than that means flicker or careful spacing. Designers also had to fit scrolling backgrounds, bullets and a score into 128 bytes of RAM.
This list is unusual because many of its best-known games were never sold. As the video game market collapsed in 1983 and 1984, Atari cancelled or shelved many nearly finished cartridges, and the prototypes that survived have since been shared by collectors. Several of them are here:

Xevious: Tod Frye’s nearly complete 1984 version of Namco’s scrolling shooter; only the Atari 7800 version reached the shops.
Sinistar: an almost finished 1984 take on the Williams arcade game.
Tempest: a roughly half-finished prototype from January 1984 of Atari’s tube-shooting arcade game.
Saboteur: Howard Scott Warshaw’s 1983 robot shooter, shelved when Atari wanted to turn it into an A-Team game.
Aquaventure: a 1983 undersea treasure hunt with a spear gun, completed but never released.
Combat Two: a 1982 sequel to Combat with mazes, armour and bases, made at General Computer Corporation.

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