Catch the ennemy's cannon balls and shoot them back to destroy their tower. You can only take one at a time so avoid to be beaten while you keep one. The towers are made of metal and stone: the stone blocks are destroyed with 5 shots, while the metal ones move and allow you to destroy the balance of the enemy tower.
Use the A/D keys to move to left/right; move the mouse pointer to set direction and keep pressed the mouse button to give power to the shot; release to shoot. With the ESC key you go back to main menu.