The city wall was interesting, in a bleak sort of way. I let my imagination run wild as I imagined Ghengis Khan and his Mongolian hoards sweeping over the hills and attacking the wall, with loyal imperial troops doggedly defending each inch of it tooth and nail, to protect the motherland from the barbarians. In reality, however, it didn't happen like that. The wily Khan simply sent emissaries to different parts of the wall until he found some corrupt official he could bribe to let him over unmolested. "Any wall," he said, "is only as good as the men defending it."
Good point Ghengis! Walls don’t seem to be effective means of defense. The effort required to build, maintain and defend them greatly exceeds the force required to overcome them, and those behind the wall become complacent and inward looking. China herself became inward looking, and as the Qing royal court amused itself behind the walls of the Forbidden City, believing it had frozen time, the West progressed and moved through enlightenment and then an industrial revolution. Centuries of self-enforced isolation had left her unprepared for Western and Japanese aggression, and she is only now regaining her position as the world's number one nation.