Space Monsters

Space Monsters is a freeware top-down shooter game developed by Psonic. It is also the first installment of the titular series inspired from the influences of other shooter games: Alien Shooter, Alien Breed, Crimsonland, Space Invaders, Alien Swarm, Half-Life, Doom and The House of the Dead.

Trivia

 * The title itself is a reference of Taito's 1972 mechanical game, which is later changed to Space Invaders by Tomohiro's regime in 1978.