Rules are pretty easy. You are besieged by the enemy and have to defend your position. Your fortress is located in the middle of the screen. Enemies appear at regular intervals and shoot at you. Your aim is to destroy them before they have fired. If they have already done so and if you are fast enough, you may be able to destroy their missile before it hits you. Your fortress is strong enough - it can bear up to four strikes from each side before it gets destroyed. There is more to it - you have a shield that reflects the enemy's bombs.