More than 2500 years ago, the emperor of the Qin dynasty decided to protect his prolific provinces of northern China from the frequent barbaric invasions, building and joining several fortified fortresses. So he made an impressive series of defensive walls that stretch for over 6000 kilometres. Armies of slaves and soldiers worked for centuries following and taking advantage of the terrain. Later, the Great Wall, as it was called, was extended and fortified, until it needed an army of a million of men to defend and maintain it. Its decline coincided with the advent of the Mongols, that later ruled the widest empire of human history. They broke the wall and moved around it, conquering the whole Chinese empire. Today the ruins represent one of the most remarkable works produced by men: Chang Cheng™, the Great Chinese Wall.