Hmm...
For starters, this doesn't have sound or music, which automatically takes a LOT from the experience of any game. If you're going to fix anything, fix that. Also, I noticed that I had to manually move chose to start the next wave, but the indication for that wasn't very clear, so I thought the game have glitched for a while. Speaking of glitches, did you know that if I'm using the machine gun and then switch back to the pistols while still holding the fire key, the bullets still fire as fast as the machine gun?
Let's talk about those weapons. Do they have limited ammo? No. Then why even have the pistol? I'll never use the thing if I can use something else better indefinately. Maybe if you just had a choice between a fast-shooting (but weaker)forwards machine gun and a slow, spreading (but more powerful) shotgun, it wouldn't feel so...unnecessary. I'd also appreciate some variety in my opponents, such as ones that can attack from a distance or ones that just sprint towards the barrier. You've got a good base for a game, but you really need to improve on what you've done with it so far.