This is quite a nice little entry, I am having fun while playing it! I do like the graphics, especially the parallax background, it fits so nicely into the game. The difficulty is not too great but it feels like the soldiers are a bit too strong while the tanks are just pathetic. They do have high health but one bullet at a time is just too doable. The final boss is also not too bad once the soldiers are dealt with.
A little criticism is that all enemy spawning was not telegraphed, and thus sometimes one spawns in by surprise and caught me off guard. It would be good if there is something that foretells enemy spawning. Another thing is that for some reason full screen doesn't work for me, and it's a bit hard to play in such a small viewable area.
Anyway, good job with the game!