This turned out to be a surprise favourite of mine! Its very addictive and seeing the enemies go down quicker and quicker was lots of fun. The enemy spawning really discourages the movement which is a bit of a shame, and the enemies don't spawn on the left, but it does keep the focus on shooting the enemies quick enough.
As I upgraded more (with about 12 cannons in total), I found a bit of a performance issue, it seems like its tied to the amount of bullets I'm spawning. My best guess is that you aren't deleting the bullets fast enough / at all? But its so far into the game that it isn't really an issue and hence why you shouldn't care :D
The music is, of course, a perfect fit.
Overall, an addictive game that I could play even more with a few tweaks and additions.
Well done!