Minecraft wiki entry for Mob_farm, associated text: A basic 9×9 (not counting the walls) water tray works well for zombie or skeleton spawners, or when gathering together the output of several dark areas. Simply put water sources at the corners of the square, and they flow to drop the mobs into a hole in the central square. For spiders or iron golems, you must expand this to 10×10, which allows for a 2×2 hole in the middle (for spiders, you must also line the chute with glass).