Minecraft wiki entry for Mob_farm, associated text: If there is not enough space for larger designs, this might be used. Its small size of 10x10 or 20x20 (inner area) makes it able to be incorporated into larger construction projects without trouble. It uses four water source blocks, one in each corner, with the water flowing around 3x3 squares of building material to the central hole. The parts of the water between the wall and the blocks is closed over to provide more spawning area.