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.

Ports were developed for contemporary game systems, most of them by either Atari, Inc. or its software label for non-Atari platforms, Atarisoft.

Landers pick up humans and attempt to carry them to the top of the screen at which point they turn into fast-moving mutants.

[4] Initially, Jarvis spent 3–4 months developing color variations of Taito's Space Invaders and Atari, Inc.'s Asteroids.

The team experimented with recreating the game with pixel graphics, but also abandoned it because they felt the gameplay lacked enjoyment and visual appeal.

[11] By July, development was behind schedule and Jarvis's superior began to pressure him to finish the game in time for a then-upcoming trade show, the AMOA, in September.

Jarvis spent several weeks creating the astronauts, which his boss felt should be omitted if the process didn't speed up.

The Mutants added a rescue element to the game that Jarvis believed made it more interesting to players and encouraged them to continue playing.

[11] By September, the game was still unfinished, and almost every Williams programmer assisted in meeting the deadline for the AMOA trade show.

[10] The developers, however, forgot to create an attract mode (an automated sequence designed to entice an audience to play) and high score system for the game, and began working on them that night.

DeMar coded the attract mode, Dussault and Dicker created the high score table, and Jarvis playtested and fixed bugs.

The chips were put in backwards, causing an electrical short when the cabinet was turned on, so the team had to quickly burn a new set of EPROMs.

[15] A pack of three AA batteries provide power to save the game's settings and high scores when the machine is unplugged from an electrical outlet.

[24] The Amusement & Music Operators Association (AMOA) later listed Defender among America's six highest-grossing arcade games of 1982.

[4] At the time of its release, Stan Jarocki, director of marketing at then-competitor Midway Manufacturing,[Note 1] described the game as "amazing".

[32] The magazine was more critical, stating that "the game's appeal does not justify its unreasonable cost" of being shipped on ROM cartridges.

[27] David H. Ahl of Creative Computing Video & Arcade Games said in 1983 that the Atari 5200 version was "a substantial challenge to the most seasoned space gamers".

"[35] Next Generation ranked the arcade version as number 13 on their 1996 "Top 100 Games of All Time", saying that its balanced difficulty makes gamers keep coming back for more instead of giving up.

"[37] In 1999, Next Generation listed Defender as number 23 on their "Top 50 Games of All Time", commenting that "despite exceptionally complicated controls, gamers fell in love at first sight.

[39] In 2008, Guinness World Records listed it as the number six arcade game in technical, creative, and cultural impact.

[13] Matt Barton and Bill Loguidice of Gamasutra stated the audio-visuals and gameplay's depth balanced the excessive difficulty.

[41] Players have competed to obtain the highest score at the game and the longest play time on a single credit.

[7][45] It was authenticated by the facility and the many witnesses that watched along with the press of Atlantic City; the media attention spurred other players to attempt the same feat.

[46][47] One bug, related to how the game keeps track of scoring, allows players to earn a large number of "extra lives".

[48] Professor Jim Whitehead described Defender as a breakthrough title for its use of full 2D motion, multiple goals, and complex gameplay that provides players with several methods to play.

He commented that its realism and technological advances pushed developers to create more popular games, citing Gorf and Phoenix as examples.

[9] The success of Defender prompted Williams to approach Vid Kidz, who originally wanted to create a new game.

[59] In 1997, Tiger Electronics released a handheld edition of Defender with a grayscale LCD screen, which doubles as a keychain ornament.

[60] Home games that copied Defender's design include Gorgon (1981)[61] and Repton (1983)[56] for the Apple II; Alien Defense (1981) for the TRS 80 Model III;[62] Chopper Command (1982) for the Atari 2600; Protector II (1983) and Dropzone (1984) for the Atari 8-bit computers;[56] and, for the BBC Micro, Defender (1982) was renamed to Planetoid (1984) to avoid litigation.

[56] Later games influenced by Defender include Datastorm (1989) for the Amiga and Fantasy Zone (1986) for arcades and a variety of home consoles.