Many popular DOS games were ported from video game consoles:
Video game consoles generally have wildly different hardware than PCs. They have different CPU architectures, graphics modes, and specialized chips for video, sound, and effects that were specifically targeted by the game developers who programmed these games. You can actually notice the difference when playing a port versus a game developed originally for MS-DOS (such as Commander Keen).
Therefore, I have always wondered about the "runtime" environments these DOS ports shipped with. I assume these games were not completely recreated from scratch when they were ported to DOS. At least they could reuse the artwork and level files. But to what extent was the game's original engine reused, or perhaps even emulated, to make the game playable on DOS computers?