Before Wolfenstein and Doom id made a game more devilish, more monstrous, more nefarious than you could tentatively imagine: Commander Keen. You played as an 8-year-old boy flung across the galaxy with his pogo stick to battle the alien forces massing for an invasion of Earth. At least in Doom you had a shotgun.
The source code for that ancient platformer’s been released for anyone to take a look at.
The release has resulted from a crowdfunding campaign that sought to get the lost episode of Commander Keen on sale in digital stores. While there’s a Complete Keen Pack available on Steam it misses two episodes that weren’t published by Apogee, so they can’t be included. The campaign raised the money to buy the rights to the code and game license, allowing the new owners to sell the episode through Steam and GOG, as well as releasing the source code for free.
You’ll still need to buy a copy of the game to get hold of the assets but you can start looking at the code immediately.
The campaign organisers are trying to get the Commander Keen: Keen Dreams through Steam Greenlight at the moment, helping it on its way will get you access to those art assets all the sooner.