Minecraft wiki entry for Walls_and_buttresses, associated text: A ha-ha is a concealed barrier which stops livestock wandering onto the grounds of an estate without the need for obtrusive fencing. Because these are sunken walls they quickly become invisible when viewed from a distance. In Minecraft, making them half a block lower than the adjacent earth conceals them very effectively. Note that some extra non-historical changes would be needed to stop spiders as well.