I read that the game engine was made in Pascal to imitate Quake style graphics. the fact that this is a custom game engine makes it so impressive and gameplay is good too, the theme of the game is unique and hence it stands out among most similar games that try to recreate the feel of Quake, hence I like the theme.
Always wanted to beat the remaining two episodes, but shareware version did not have them. This is one of the most unique sci fi platformer from back those days and it is totally awesome. If you have played Commander Keen, I think you will surely like this gem too . . . There are also some hidden easter eggs related to it . . :D