Demon Attack is a two-dimensional shooting game for early game systems (e.g. the Texas Instruments 99/4A) where the aim is to defeat waves of aliens with a laser cannon. Unlike other early shooting games, such as Space Invaders, Demon Attack does not give the player any obstacles to hide behind and shield the player from enemy attack. Also, unlike Space Invader-type games, there are fewer aliens on screen at any given time but the alien sprites are bigger and more elaborate. The cannon also moves much faster than in Space Invaders, with a smaller footprint making it easier to evade enemy fire.
The following is primarily based on the Atari 2600 version. Other systems are likely to have differences, either major or minor, due to the quality (or lack thereof) of some ports.
The player controls a single cannon at the bottom of the screen which can move left and right. Up to three Demons will warp into existence and float around, with the lowest Demon dropping missiles. Destroying a Demon will cause a new one to warp into the old demon's place until the level's 'stock' of Demons is exhausted, at which point destroying the remaining few Demons will clear the stage.
As the stages progress, the aliens become faster and change appearance, and some will split in half when struck instead of being destroyed. If one half of the lowest Demon is destroyed, the other half will fly down towards the cannon in a kamikaze run. Thanks to its small size, it can be difficult to hit before it reaches the ground, though it can be evaded if there is enough room to the side. Once both halves are destroyed, the middle Demon will float down to take the place of the lowest, and if one half is destroyed the remaining half will immediately fly down towards the cannon. Later demons also drop missiles that follow them, letting them sweep their fire across an area when they move.
The first run of cartridges stopped gameplay after the 84th wave, supposedly by design. Every run of the cartridges thereafter allows the player to 'wrap' the score and gameplay continues until all lives are lost, though the difficulty does not increase.
As is the case with many older video games, there are a few variations available that affect gameplay.
There are two methods of playing two-player - first, alternating play stage-by-stage to compete for score, and second, co-operative play with the cannon changing color to represent the controls switching between first and second player at pre-set intervals.
The other option allows the cannon's shots to be guided by the motion of the cannon itself, much like the attacks of the later Demons. This is less useful than it sounds as it means you have to position yourself underneath the lowest demon to shoot it, placing you directly in its line of fire.
Atari itself filed a lawsuit against IMagic, claiming it was too similar to Phoenix. IMagic and Atari settled out-of-court, and Demon Attack went on to sell quite well.