Good game
I think you could have used a longer music loop, or possibly even a couple of songs. A sound off or a volume button would also be nice. The tempo of this game is also on the slow side. I ran into a glitch where the shots would go right through the tank. The thing that bugged me the most about this game was that you had to roll over that little question mark to get your health, power, and time left.
I liked the shield, something original is always nice. Overall, good work!