Minecraft wiki entry for Tips_and_tricks, associated text: If you have enough gold and some redstone for powered rails, you can craft those plus a few minecarts, and set the carts spinning around the perimeter. Any mobs that come close enough to the tracks will be scooped up by the minecart and taken for a ride. This trap can be modified with one-block-high tunnels to suffocate any mobs in the cart. The player can also ride the carts by the player - they will travel fast enough that most mobs can't attack him or her. This, however, takes much more skill to use, and can still trigger creeper explosions.