You have to destroy the enemy castle with your cannon, so think well to find the ideal shot trajectory and power. To destroy the castle you need to hit 13 times. Remember that you just have 7 seconds to aim and fight, after what the cannon will shoot by itself and give the turn to your rivals. And also remember that the wind affects your shot!
Arrow keys up/down to set the angle of the shot; arrow keys left/right to give more/less power to the shot. Space bar to shoot. P key to stop the game. If you want to pass to switch between 1 player and 2 players mode, click at the computer icon. The second player aims with the W/S keys and sets the power of the shot with the A/D keys.