Fun, but room to improve
Overall, a fun submission. It needs a few tweaks in my opinion to make it to a better grade than a 70.
First, adding a reload button. It's irritating to have a half-full magazine and to have to crack off those rounds to refill in a lull.
Second, widen the view field so that it feels more like you're picking them off at a range. You could even make the targets smaller and more numerous if you slowed them down. This would let you keep the same screen size but feel more like sniping.
Also, let the enemies stop in any given range of spots. When getting overwhelmed it was possible to just aim at the same spot at the base of the tower and get rid of them all.
Keep trying, and you'll get something 10-worthy soon.