Apogee (now 3D Realms) and its shareware model. The second trilogy sold fewer copies, which was blamed by id and Apogee on its split into two parts, and the 2001 game received mixed reviews. The MS-DOS games have been re-released in several compilation packages, and all but the sixth episode are still sold through modern emulation releases on platforms such as Steam. References to the series have been made by dozens of other games, especially to the Dopefish, an enemy in the fourth episode, which has been termed one of the video game industry's biggest in-jokes. An active modding community has grown around the series, producing editing tools and unofficial sequels. Titles Games 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. Commander Keen in Keen Dreams is the second game of the series, released in 1991 as a single episode; it is sometimes referred to as episode 3.5 of the series. It was developed for MS-DOS by id Software, which had been founded by the Ideas from the Deep team after the success of the first game, and published through Softdisk as part of an agreement between the companies as the founders of id had been employees of Softdisk when they developed Vorticons. Id used the game, which stands outside of the general continuity of the series as a "dream" episode, as a prototype to test ideas they wanted to use in future games, such as a more advanced art style, parallax scrolling, and gameplay changes. In 2013 a port was developed by Super Fighter Team for Android devices, and a 2014 Indiegogo crowdfunding campaign led to the original source code being released under GNU GPL-2.0-or-later and a release by Hard Disk Publishing through Steam for Microsoft Windows and Linux in 2015,