Before Id software took over the world of PC gaming with Wolfenstein 3D and Doom, they broke technological ground by introducing 2D scrolling graphics to PC games!
In a time when console hardware was far ahead of the PC in terms of gaming, 2D PC games consisted of single screen games where moving to the edge of the screen would teleport you to another screen or pause all the action to play a static scrolling animation. PCs just weren’t capable of hardware scrolling like the consoles of the time and scrolling like in consoles was thought to be impossible on PC.
Enter Id Software who, using clever programming tricks, managed to create a game engine that could actually scroll just like in consoles games! After failed pitches to sell this engine, they decided to make their own game using said engine.
The first entry in the series being released in late 1990 for MS-Dos, Commander keen went one to receive 2 more sequels on the same engine before receiving an engine upgrade along with 3 more sequels running on said engine and 1 spin-off for a total of 7 games (and 1 Gameboy Color game released much later from different developers which wasn’t very good…)! You play as Billy Blaze (a boy genius and descendant of the Wolf 3D protagonist), who must run and gun his way on strange planets to defend the Earth, the Galaxy and beyond.
Although the game has never received any official ports to other hardware platforms, you can buy emulated versions of the original MS-Dos games on Steam which I recommend as these games are real classics.