The Cannon is a weapon added by Balkon's Weapon Mod. You can place the cannon wherever you want, and mount it. When mounted, a cannon can be turned in all directions. To load the cannon, make sure you have gunpowder in your inventory, and right click on the cannon with a cannonball. Wait a few seconds, and then hold down the left shift (or space) button to fire! In the properties file you can disable block damage.
NOTE : Pointing the cannon in a random direction and firing it can cause the cannonball to hit the ground beneath you, resulting in a painful death or more seriously, the damage and destruction of property (if you happen to place it on your castle).