If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
To pass each level your task is to destroy the enemy's main tank. The more enemies you kill, the more money and points You'll be able to earn and You'll be able to improve your ranking. If you did not get enough money needed to upgrade Your tank,money will not reset and you can continue to gather up enough money to upgrade your tank.
Arrow keys to move.
Space bar - switch weapons.
Ctrl - show or close map.
Mouse - aim and shoot.