Haha is right. A haha is a kind of "hidden" barrier that you find on large country estates. When Lord Hobnob looked out of his windows across his country park, he didn't like to see a fence or wall that spoiled his view. The cattle/sheep still had to be kept off his lawn, so the answer was to dig a ditch where a fence might go, and reinforce one side of it with a wall that did not project above the land. From the house, this wall was invisible.