Set the catapult's power and angle appropriately, and try to hit the castle in the least number of shots possible. Take control of a catapult and shoot boulders at the castle to destroy it. Set the angle and strength so that the boulder hits the Keep of the Castle to complete a level. You'll get bonus points if you hit the flag.
You can also hit other objects and enemy knights to gain points. But, avoid hitting your own knight or you'll lose a life. The game will be over if all your knights are dead. Keep and eye on the wind direction when you take a shot.