You're trying to besiege about twenty castles with the help of your cannon. Of course the lords of these castles don't want this to happen! They defend themselves by shooting back with their own cannons. Therefore, try to eliminate these cannons by shooting at them. Hit the cannons twice to eliminate them completely. The hostile cannonballs can hit your cannon directly, but they can also sweep away the ground around it, which can also be quite tricky! So think hard about your attacks, and aim carefully!
Use the mouse to aim and click to fire.