No one can tell precisely when the building of the Great Wall was started but it is popularly believed that it started as a military fortification against attacks by tribes on the borders during the earlier Zhou Dynasty in the 7th century BC. It was not until the Qin Dynasty that the separate walls, constructed by the states of Qin, Yan and Zhao kingdoms, were connected to form a defensive system on the northern border of the country by Emperor Qin Shi Huang. After the emperor unified the country in 214 BC, he ordered the construction of the wall. It took about ten years to finish and stretched for more than 5,000 kilometres. The wall not only served as a defence in the north but also symbolized the power of the emperor.

The Great Wall was renovated from time to time after the Qin Dynasty. A major renovation started with the founding of the Ming Dynasty in 1368, and took 200 years to complete.

Armies were stationed along the wall and fires were lit to signal approaching enemies. This great defensive wall worked well and invaders only succeeded because of weakness in the government and the poverty of the people. They took advantage of rebellion from within China and took over without long wars.

Over the past few centuries, the Great Wall has served as a source of building materials for local farms and villages. Aerial photos show that in sections, only the top battlements show -- the center of the wall has filled with sand and silt. The isolation and harsh conditions make restoration slow and difficult.