Nice job =)
Visuals: Awesome-2.5
Animation: Awesome-2.5
Audio/Sounds: Good-2.0
Game Play: Okay-1.5
Total: 8.5/10
I've seen this idea come up a few times in Newgrounds. I don't remember what the name of the game was but it was exactly like this, only it was an archer firing instead of a cannon. So I couldn't give you the points for game play originality. I saw a few minor bugs but I won't list them since the reviews before me already covered that. Also, when I got the ice bullets, it sucked very bad damage-wise and it cost me the game. I suggest a weapon toggle feature in which you can switch which type of bullet you want to use. Lastly, the upgrades and variety of soldiers were very few and the upgrades were not that hard to get since you can just sit and pick off soldiers while you watch your money rise. If you are making a sequel, don't make the money rise by itself and make the upgrades a little harder to get, like raising the prices a little more.
Now for the good things I saw ^_^. The animation was smooth and the physics was about right. Though the variety for the soldiers on our side wasn't very extensive, I liked the new challenges that I had to face on every planet. The audio was crisp and clear but a bit repetitive, the soldiers didn't pull anything stupid, the upgrades worked, and the concept was well-executed.
Great job. Keep working at it! Cheers ^_^