Not bad, could use more work though...
The plasma bombs appear to be broken. I've fired about 15 of them, and none have destroyed anything. They appear to just pass through enemies whenever i shoot it. Please fix this.
Also, I think it would be nice if the main gun upgrades actually had different shooting graphics. Maybe even just different colors, or a thicker beam would be nice. The AI on the additional turrets is well done, as is the increase of difficulty as levels progress. I'd like to see more upgrades, however, because I found myself maxed out long before finishing the game. One option is to prevent purchase of a high level cannon until all preceding cannons have been purchased. This stretches out the upgrade system a bit. Otherwise, it's much more cost efficient to save up money and skip a couple main gun upgrades.
Personally, I rather enjoyed the game. It's clear you put a lot of work into it. I think you have talent, and could do even better with future efforts.