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The object of the game is to defend various cities from an unnamed enemy's attacks. Each round of the game takes place in one location (e.g. "France"). The assaults consist of waves of planes, helicopters, night bombers, and MIRV bombs. Similar to Space Invaders, each attack wave moves back and forth across the screen, getting faster in successive waves.
The games distinguishing features include:
- Each round begins with a spoken mission brief which begins with the words "Red Alert! Enemy aircraft approaching!", followed by details of the game objectives for the round. This is one of the earliest uses of speech in a video game.
- Landmarks from the city being defended are shown in the background of the gameplay. For example, during the "France" round, a depiction of the Eiffel Tower is shown.
- Some rounds take place at night. In these rounds, the game area is illuminated only by two spotlights, which the enemy can destroy.