The Great Wall of China is the most famous structure in the People’s Republic of China. These castle wall ruins are designated a World Heritage site by UNESCO. The structure began with the building of fortifications by each state during the Spring and Autumn Period and Warring States Period (around the 7th century BC) to defend against intrusion by foreign enemies and other ethnic groups. When Qin Shi Huang unified China, he had a wall built to connect each fortification in the North and extended it to the West. Time passed, and during the Ming dynasty (the 17th century), the wall was repeatedly reinforced and repaired using advanced design technology. The wall is said to have extended from the Shanhai pass in the East to the Jiayu pass in the West, with a total length of 8,851.8 km. Most of the wall that remains today was built during this era.