Minecraft wiki entry for Defense, associated text: Place a minecart with TnT on the regular rail and push it off the ledge onto the powered rail. Repeat this with the other minecart(s) with TNT. Once you are done with that, remove the rail and a block next to the powered rail. The block you remove has to be one of the blocks that the powered rail is facing. When you are done, it will look like this: