The Great Wall was heavily fortified, serving as the military power line of the Silk Road as early as the Han Dynasty (206 BC-AD 220), when caravans of horses and camels transported men and women, trade wares and silks, plants and animals, technologies and religions.
Perhaps no other exchange of information was as profound as what occurred leading up to the Golden Age of Chinese philosophy (Tang Dynasty, AD 618-907), when Indian Buddhism flowed into China along the trade routes of the Silk Road, carrying information and ideas that changed how the Chinese, Japanese, and Koreans think.
For example, ancient Buddhism and art, illuminated with Greek and Persian influences, flourished in Western China, a sign that east-west cultural communication and collaboration is much older, and much better developed, than previously thought. (Visit my Silk Road and Pakistan pages to learn more about Grecco-Buddhist culture and China, or see the works of Sir Aurel Stein and Sir John Marshall).