sentences:  As the greatest defense-structure project in the history of human civilization, it dates back more than 2,000 years ago to the Spring and Autumn and the Warring States periods - huge in its scale and grandeur.There are more than ten sections of the Great Wall open to tourists, including the passes, blockhouses and beacon towers at Badaling in Beijing, Laolongtou in Hebei and Jiayuguan Pass in Gansu.Grottoes filled with precious murals and sculptures are concentrated along the ancient Silk Road in Gansu.