S1, E4: China's emperors built the Great Wall to keep their kingdom safe from the hostile barbarians to the north. This is a land of warrior tribes, bizarre wildlife and extreme weather, but also of vast and breathtaking evergreen forests, grassy plains and sweeping desert dunes, rich with history. The legendary Silk Road drew traders and their camels across the deserts in search of fabulous wealth, and fierce Mongolian horsemen conquered the known world. Today, nomadic tribesmen still race horses and hunt with golden eagles, while tiny hamsters and Asia's last wild horses struggle to survive in the world's most northerly desert.