In ancient China, the people desired security from the barbarians to the north. The result? The Great Wall of China. It’s 30 feet high, 18 feet thick, and more than 2400 km (1500 miles) long! The goal was to build an absolutely impenetrable defense—too high to climb over, too thick to break down, and too long to go around. But during the first 100 years of the wall’s existence, China was successfully invaded three times. But it wasn’t the wall’s fault. The barbarians never climbed it, broke through it, or went around it. They didn’t have to. All they did was bribe a gatekeeper and march through an open door.