Commander Keen in Invasion of the Vorticons is the first game in the series, released on December 14, 1990, for MS-DOS. It is divided into three episodes: "Marooned on Mars", "The Earth Explodes", and "Keen Must Die!", and the game was published through a shareware model whereby "Marooned on Mars" was released for free, and the other two episodes were available for purchase. It was developed by a team calling themselves Ideas from the Deep (or sometimes IFD Software), consisting of programmers John Carmack and John Romero, designer Tom Hall, artist Adrian Carmack, and manager Jay Wilbur, and published by Apogee Software. The game, like the rest of the series, is a side-scrolling platform video game.
Source: Wikipedia