No one can tell for sure when the building of the Great Wall was started，but it is popularly believed that it originated as a military fortification against intrusion by tribes on the borders during the earlier Zhou Dynasty.Late in the Spring and Autumn Period，some states extended the defence work to prevent the attacks from other states. It was not until the Qin Dynasty that the separate walls were connected to form a defensive system 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 the wall stretched from Gansu Province to Jilin Province.The wall not only served as a defence in the north but also symbolized the power of the emperor.