Minecraft wiki entry for Wither_skeleton_farming, associated text: All mobs spawnable in nether fortresses always spawn in specific locations. These locations seem to be at random crossroads.[verify] If a crossroads is chosen, mark out a five by five area in its center that will be your drop pit to the kill zone. Then take all sides of that 5 x 5 center hole and build out using nether brick blocks to make a flat floor with at least three blocks between floor & ceiling if there is one.