Medieval Wall Cannon by apengstrom, from Kingdom of Artania Server. Visit our website at www.kingdomofartania.com/

To load, fill the front with TNT on each side (via the piston mechanism at the top of the wall), and 1 TNT in between the empty space in the wall front at the top. Press the button to the right first, then the button on the left between 2-3 seconds later. You have to time it right, or you'll blow up the front of your cannon with your ammo! :)