Inspired by Taito's earlier electromechanical machine Space Monsters[4] and Tomohiro's interpretation of alien descriptions in War of the Worlds,[1] the game itself resembled an adaptation of the two. In this video game version of the game, the player controlled the motions of a movable laser cannon that moved back and forth across the bottom of the video screen. Rows and rows of video aliens marched back and forth across the screen, slowly advancing down from the top to the bottom of the screen. If any of the aliens successfully landed on the bottom of the screen, the game would end. Although the player's laser cannon had an unlimited supply of ammunition, it could only fire one shot at a time.Meanwhile, the aliens would shoot back at the player, raining deadly rays and bombs that the player would have to dodge lest his or her cannon be destroyed. Players could also move the laser cannon under one of the shelter blocks, so that they could absorb the enemy shots until they are worn through. The player's cannon could be destroyed up to three times (the player had three lives), and the game would end after the player's last life was lost. Occasionally a bonus spaceship would fly across the top of the screen which the player could shoot for extra points.
As the player destroyed an increasing number of aliens, the aliens began marching faster and faster, with the lone remaining alien zooming rapidly across the screen. Shooting the last alien in the formation rewarded the player with a new screen of aliens, which began their march one row lower than the previous round.
Another important tactical element of the arcade game is that it is impossible for the players' spaceship to be harmed by an invader firing a missile from the lowest line on the screen before the invader lands.
