You're on a mission to defend a Mark 16 Starcruiser from enemy alien ships that teleport in and start firing missiles. The goal is to destroy the enemy ships until reinforcements arrive from Earth. The game takes place in the bottom third of the screen, where you can move in any of the eight directions using your ship. To fire, simply press the button. The aliens will try to destroy you with neutron missiles that can be deadly upon impact. Be aware that as the battle progresses, they'll use more and more laser-directed heat-seeking proton missiles.
Scoring varies depending on the level of the alien and what you hit. Level one aliens are worth 125 points, while level three aliens are worth 275 points. Shooting an alien missile is worth 50 points. You can reach levels two and three at specific point totals: 10,000 for level two and 40,000 for level three.
The game also features difficulty switches that affect your ship's speed: A slows down the shots, while B speeds them up. There are different variations of the game, including solo and two-player modes, as well as games with limited or maximum heat-seeking missiles.
Enemies start at level one in some games, but can progress to higher levels in others. The day-night cycle also varies depending on which game you're playing, with daytime in the first 12 games and nighttime in the remaining ones.
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