The ships of ancient times are coming by sea to destroy your kingdom and invade your castle! Stop them in their tracks with all manner of projectiles to ensure that they don't accomplish their goal!
A different fleet comes to attack at every round. Take them out as fast as possible in order to earn extra money. Upgrade your cannon as you go and be sure to buy ammo for it. And remember...its not the size of your enemy that matters, its the size of the cannon you're pointing at him!