Back when it was still used, there were (and still are) towers every few hundred feet in which a small bunch of soldiers would keep watch (usually 3-4) in shifts. When danger was spotted, they would light a signal fire and troops would usually be sent from a garrison (usually near a gate) to repel the attack.
Pics of Gate: http://farm4.static.Flickr.com/3662/3403036206_e95efd76eb.jpg
3 crumbling towers: http://www.globusjourneys.com/Common/Images/Destinations/great-wall.jpg
This however failed as usually a treacherous general would open the gate:
Towards the end of the Ming Dynasty, the Great Wall helped defend the empire against the Manchu invasions that began around 1600. Under the military command of Yuan Chonghuan, the Ming army held off the Manchus at the heavily fortified Shanhai Pass, preventing the Manchus from entering the Liaodong Peninsula and the Chinese heartland. The Manchus were finally able to cross the Great Wall in 1644, when the gates of Shanhai Pass were opened by Wu Sangui, a corrupt Ming border general, after being bribed. The Manchus quickly seized Beijing, and defeated the remaining Ming resistance, to establish the Qing Dynasty.
Traitor to China: http://en.wikipedia.org/wiki/Wu_Sangui
The great wall of china is there to protect the country. ^.^
The great wall, mountains, and armies
to protect china to protect china
purpose of construction of great wall of china to protect from attacker at china
China built the Great Wall to protect their country from invaders.
China's great wall was built to keep out the large invading armies of the Mongol people. The Mongols had pillaged China and raped their woman for centuries. The Great Wall put a stop to it.
well, there was a war in China so they build the wall in China to protect China
To protect china from invaders!
It was built to protect and isolate China.
they came up with the great wall of China by finding ways to protect them from intruders.