Mars Attacks!

Clone of Space Invaders which has nothing to do with the movie of the same name by Tim Burton. Plot: in 2997, a group of astronauts discovered human traces on Mars, and then a Martian colony. Martians prepare an attacked against earth. Fortunately for humanity an army is created and sets off for Mars to destroy its inhabitants. Apart from the plot this is still Space Invaders, with pretty poor physics and collision detection.