Minecraft wiki entry for Defense, associated text: Liquid walls are walls, which are coated on the outside with a layer of water or lava. While they can allow mobs to enter the liquid and float over the top (undead mobs will sink), they can be used as a damage dealer by drowning or burning them. They can also prevent creepers, ghasts and player-made TNT cannons outside the walls from breaking the wall, and water also prevents endermen from taking blocks out of the wall. While liquid walls are much harder to break than regular walls, though, it is still recommended to build an overhang structure to prevent climbing mobs.