good idea
Good idea but needs a lot of work. The recoil is massive when you charge up the cannon. Most of the times it's the recoil that ends up pushing me out of the stage. So this kinda makes the whole powering up to shoot thing redundant. You are better off shooting many 'low power' shots.
Because of the recoil, I thought the level with the two gates is ridiculous. And I quit playing there as it was just frustrating. There some some oddities with the htTesting also, I was hugging the gate and one of the enemy projectiles still hit me and pushed me off. There was also some weird event at the exchange point where the two gates meet that for some reason also pushed me off the stage.
Like the guy below says, it's the same game every level. The teleporters don't really spice up the game much. You should add some point/money system and let us buy upgrades for the tank after each level. I think pretty big on the to-buy list would be reduce the recoil :P. Other upgrades can be more power to the shot, the projectile speed, the tank speed, the tank endurance (how much it's pushed when hit by enemy) etc. etc.
As it is the game falls a bit short as it's frustrating and not rewarding. Good idea for a game though. You can improve it a lot if you plan to do a sequel.