From Pitch Mobile:
Use your shield to protect your base from attacking tanks and repel their own projectiles back at them! Earn money by killing tanks and buy upgrades between levels to stay ahead of the game. You will receive one bonus point every 3, 4, and 5 levels in Sergeant, Captain, and General mode respctively... don't forget to spend them! Kill the boss tank every 10 levels before time runs out for extra bonus points. The score is reset to 0 when continuing from a saved game. For those asking for keyboard control of the shield: I added that at one point but the shield was either too slow to get around quick enough or too fast to be accurate... there's just no beating a mouse for that kind of control. - The 'return to main menu' option in the pause menu was added by request.