From the name of the game itself Crush the Castle you will get the idea of this game.
There is an order from the King to crush all the resistance. With the use of different weapons available such as rocks, arrows and any available weapon that can be used to reach your goal is to crush the castle and break it down and if you destroy more you will get more additional points. To start the game you need to click anywhere to be able to start the trebuchet launch and another click to be able to release the projectile.
So good luck in your mission and make the King and the kingdom proud.