Minecraft wiki entry for TNT_cannons, associated text: This trap is very simple: Go up high, build a platform, place dispensers downwards, set a clock, and watch as your enemy's base is obliterated. Simply block-pillar up to a good height. Do not build too high, as the TNT would explode in midair, uselessly. Build the platform directly over the target. Place dispensers and fill them with TNT. Set a Clock circuit to trigger the dispensers, but be careful! Setting the speed too fast will have the waves of TNT literally launch upwards and destroy your cannon. You can mine the dispensers later. As to the problem of getting back down, you could use water, but Ender Pearls could come in handy.