Minecraft wiki entry for Defense, associated text: Moats are constructed identically to ditches, but they are additionally filled with either water or lava, with the specific liquid depending on whether harming the mobs or obtaining their drops is required. A water-filled moat, if constructed correctly, can be built into a drowning tank to kill mobs. The down side to this is that a player needs to be nearby to lure mobs into the water, and the AI for mobs makes it hard to do this. On another note, water-filled moats will still slow mobs down allowing them to be easily picked off with a bow and some arrows, after which the player can swim out and pick them up, or have water currents deliver them to a pick-up point.