Zap!

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A brief history of shoot 'em ups

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Shoot 'em up
Level (video gaming)
Power-up
Boss (video gaming)
Spacewar!
Space Invaders
Defender (game)
Tempest (arcade game)
Xevious
Commando (game)
Gradius
Space Harrier
Ikari Warriors
Uridium
R-Type
Xenon (video game)
Batsugun
Geometry Wars