this could eaisly be so much better
first off you need to make some kind of graphic effect for registering hits, weather its the guys flashing, or gibs flying off. the same goes for the player you cant tell your getting hit if your not watching your life.
Next, you can easily walk through the whole game with out firing a shot. you need some kinda screen lock effect so you cant move on untill all the guys are dead.
also the invisible walls are ghey, put up some fences or some shit to add to the enviroments
other wise it was pretty good, new weapons would be cool too, but you've built a strong base engine so the next one should kick ass.