This is a photograph of the Great Wall of China. Most of the existing Great Wall was built during the Ming Dynasty (14th to 17th centuries) to protect the northern borders of the Chinese Empire. The Manchus conquered the Empire in the mid-17th century and established the Qing Dynasty; they added so much more territory to the Empire beyond the Great Wall that the structure was no longer needed to protect the Empire from outside invaders. The wall stretches for many thousands of miles linking fortresses. Signal towers were used for communication.