It's an okay game, certainly something I'd see myself playing if I wanted to waste five or ten minutes.
I like the classical idea of this defense game, it's simple and not too complicated.
Having said that, it could do with a few features such as a shop, earning money off the enemies you kill -- simple stuff like that. But I did read your description box and I understand this is some sort of Alpha/Beta (?) if I understand it correctly, so I can't give you too much heat for that.
Couple of other issues I have is that, sometimes when you shoot, the bullets will go through the enemies, which is a tad annoying. And as you mentioned, the enemies cut off screen sometimes too.
Simple game, good time waster.