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Defender is a two-dimensional side-scrolling shooter. It occurs on the surface of an unknown and unnamed planet. The player can move his/her ship left or right and up or down. The player's goal is to destroy as many alien invader's spaceships as possible and to prevent them from abducting the humans on the surface of the planet. When the player destroys all the enemies, he/she advances to the next level. The player loses a ship when it gets hit by enemy fire or if hyperspace jumps go wrong. The game ends when the player loses all his/her ships.