Wow
That must have taken a lot of hard coding! Very nice, could use for a little more difficulty...suggestions:
Have some enemies that only you can kill, not the auto fire. That way, you always have something to do.
Make the items a little more expensive.
Have the dudes go a lot faster and be more of them in the later levels. I know you already have them a little harder, I'm just saying maybe to make them very very difficult.
Other than that, keep it up dude, great job!