My grandmother in England had carpet-holding strips (that metal strip that can be used instead of glue to hold carpet tight on a floor) installed along the top of the 6ft wooden fence around her back garden. It was invisible from either side as it ran along the top of the fence. One day when she was out someone tried to climb over the fence and caught his fingers on the strip... would have been painful if he had all his weight on them. Anyway, he didn't stick around or sue for damages but he gave the fence an almighty hammering before he left!