Hi, Mr.windser, seems you are so interested in the great wall. Let me answer your questions one by one.
1. The chinese emperors of many dynasties ordered the construction of the great wall, including the emperor Qin Shihuang, who first connected the small sections of the wall into a huge barrier to northern nomadic tribes.
2. The wall was mainly constructed by man and animal power, the wall crossed nine cities and provinces: Liaoning, Hebei, Beijing, Tianjin, Inner Mongolia, Shanxi, Shaanxi, Ningxia and Gansu.
3. The wall finally completed in Ming Dynasty (1368-1644).
4. The construction took for many centuries from Qin to Ming Dynasty.
5. It was built to keep out the attacks from the northern nomadic tribes.
6. In Qing Dynasty, the rulers themselves were the northern nomadic tribes who broke the defense of the wall and became the new ruler, so they did not use the wall anymore, and the wall finally lost its military function generally and became a tourist spot at last.
7. The wall is actually a barrier for the blend of northern minority people and people from innerland China, so the breaching of the wall won the unity of Chinese people.

Thanks for your concern and more questions are welcomed.