The beginning of this grandiose project (the total length of the wall is about 5,000 km) dates to the days of Qin Dynasty (221-207 BC).  It was never useful as a defence line since it was always possible to bribe sentries to get through.  It was however very helpful as an ancient highway for transporting men and equipment. During the Cultural Revolution, large parts of the wall were taken apart by locals for construction material.  Thanks to the tourist industry the wall is being renovated again.