Minecraft wiki entry for Mob_farm, associated text: The easiest possible design consists of a large, empty area of simple shape, with one or more holes in the ground for the mobs to drop through. The edge of each hole has to be lined with opened trapdoors or gates to trick the Mob AI into believing the hole to be solid ground. Trapdoors can also be controlled with redstone, so one could shut off the farm by closing the holes remotely.