Being attacked
What fun is a game without a little conflict; the nagging choice, the fight to the death, the doom of the cosmos? Every game needs some form of conflict to drive the player towards seeking a resolution. Our game will become a battle for points. Before, it was just shoot some targets and get some points.
Now, we will make the enemy tank shoot at the player. Every time the enemy scores a hit, we will reduce the player's score by a few points.