Haha I've finally beat the game/ demo, pretty funny. It's a little rough, though, enemies sometimes shoot at me and hit me although they are not visible, the always respawning enemies (that come with the helicopter) are a little annoying.
The level design reminds me of Duke 3D's level design - and that's a good thing.

As I've already said in my PM, I think you should remove all postprocessing effects and probably disable filtering and/ or mipmapping to achieve that old school look.