Minecraft wiki entry for Spawner_traps, associated text: Useful tips on this design by Duck//Marvin: When you make the opening in the spawn chamber for the mobs to exit, make it 3 blocks high and place a sign or slab at the top. This lets the mobs into the elevator more easily and prevents water spillage from the elevator. Also, making the transport corridors at the top 3 blocks tall instead of 2 allows the mobs to be transported faster as they do not hit the ceiling when they try to swim and get stuck.