Minecraft wiki entry for Shelters, associated text: The simplest of these is a "dry moat" or trench, a couple layers deep so that skeletons and zombies can't get across. This can provide you with a reasonably safe area that is outside. This can be filled with water to push the enemies to a mob grinder, or simply filled with lava which has more dramatic and obvious effects. Please note: It might not be a good idea to use lava when having a shelter built out flammable blocks such as wood, wool or wooden planks. Tower shelters would do well to have a three-block deep water moat to break the player's fall so they can safely fall to the bottom.