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Science-fiction. Commonly including things like aliens, advanced technology, and settings of outer space or distant planets. 

Shoot 'Em Up or shmup games are a subset of shooter games where the player continually moves forward while waves of enemies enter the screen. Both shooting and dodging are critical to advancing. See also Bullet Hell.

A perspective where the camera is placed directly above the player. Common in 2D games, but can also be used in 3D games.

Shooter games with a top down, birds eye view of the characters surroundings.