Minecraft wiki entry for Drowned_farming, associated text: The drop shaft is a vertical tunnel through which mobs fall onto the killing platform. The walls of the shaft need not be solid for the whole distance, it's enough to close off the sides of the shaft and leave 6 blocks of space open in the front, if you want to view the drowned falling. Drowned can drift while falling, so an open drop with no walls might prevent the drowned from falling onto the killing platform at the bottom.