Shoot the enemies. There are 5 levels. The enemies are: Jeep-1 hitpoint, Tank-2 hitpoints, Super Tank-3 hitpoints, Mega Tank-4 hitpoints, Micro Tank-5 hitpoints. You can buy extra health and cannon upgrades. Only three upgrades. The game will automatically pause when you buy something. The smaller the enemy that gets to your tank, the more damage it will do.
