It's Duke Nukem 3D, but with different weapons, enemies and levels, also Duke is a ninja now. Overall this is one of the most advanced build engine games ever, there are vehicles, turrets, underwater levels, multiple fire modes for weapons and it doesn't feel all that different to play compared to a modern shooter (for a build engine game anyway).
It runs great on DOSBox and is free, at first I just wanted to play this for a bit to see if I would like the redux version but I ended up beating it instead because it's that good.