Minecraft wiki entry for Witch_farming, associated text: Look around the farm for any bare and exposed blocks without Redstone dust on it and that are not holes in the farm's structure. If you did the job right with your layers, you should have no air gaps between layers and only the front of the farm to seal up. For all exposed full blocks, place any kind of button on top of the block to prevent spawning there. Once all 18 buttons are put in their places, go to the front of the farm and seal it up with your building block of choice. It should be a tight seal where there is no light peaking in the farm. Also make sure that the building blocks line up with the slabs on the roof.