Threshold

Threshold is a fixed shooter written by Warren Schwader and Ken Williams for the Apple II and published by On-Line Systems in 1981. Based on Sega's Astro Blaster arcade video game, Threshold introduces many enemy ship types and wave formations as the game progresses. Reviewers found the variety distinguished the game from similar shoot 'em ups. Ports to other systems were released on ROM cartridge: Atari 8-bit computers and VIC-20 in 1981, Commodore 64 in 1983, and ColecoVision and Thomson computers in 1984. An Atari 2600 adaptation was published by Tigervision in 1982. It is unique in allowing the player's ship to move vertically.