Man, I loved that game so much. And it was super easy to build and substitute your own levels, sprites, background music, sound effects, even the mechanics of the game itself, as much of it was script-based and the game came with editors for everything. You could practically write your own game on top of the existing engine and weaponry.
It also was the only game on my 486DX with its own minimalistic config.sys because it needed a mind-boggling 6800kB of free RAM.