What an awesome execution! I really love the core gameplay that you have going for yourself. I think the game could be super cool if you juiced it up a tad bit more and that would make up for the very simple graphics. I managed to score 15k so far and I'll def. play a bit more.
I do want to mention that it's very unclear to me what turret #2 does and how it's used. I also thing that having a camera that zooms out a bit if you reach the edge of the screen would help the game quite a lot. Especially if you are trying to dodge things in the middle of the bottom.
Also export that to webgl for all the mac/linux people ;)