Homeland Security: To keep out Islamic State terrorists, the Saudis are building a 600-mile-long barrier — complete with five layers of fencing, underground movement censors and radar cameras — on their northern border. If they can do it ... Good fences, it is said, make for good neighbors, which on a grand scale was the intent of the Great Wall of China in antiquity and the Israeli fences of more recent vintage. Both were designed to keep out intruders and to make sure all guests were invited guests. Unlike the Berlin Wall, they were built to keep hostiles out, not...