Commander Keen 1: Marooned on Mars

Commander Keen: Marooned on Mars is the first episode of the trilogy that put id Software on the map in December 1990, well before Wolfenstein and Doom. You play Billy Blaze, an eight-year-old genius who builds an interstellar ship out of household junk, dons his brother's football helmet and becomes Commander Keen. While he naps on Mars, aliens steal vital parts of his ship, and you must comb the Martian surface to recover them before you can fly home.

The real breakthrough here was technical: John Carmack's adaptive tile refresh allowed smooth, arcade-style side-scrolling on PC hardware that everyone assumed could only manage jerky, screen-flipping platformers. That single trick made a console-quality Super Mario-like experience possible on an EGA machine, and it is genuinely startling to see how fluid it feels for 1990. The game was released as shareware through Apogee, with the first episode free and the rest sold by mail.

Gameplay is straightforward exploration platforming: you jump between platforms, ride the pogo stick for extra height, and zap or stun enemies with a raygun while collecting coloured gems and points. It is simpler and stiffer than its sequels, and modern players will notice the limited moveset and slightly floaty jumps. Even so, as a piece of history it is essential, and the cheeky personality that would define the whole series is already fully present.