The Great Wall of China
The Great Wall of China was built by the Emperor Qin over 2000 years ago. 
It is the longest man-made structure ever built.
The main purpose of the Great Wall was to protect China from invasion or attack by northern tribes, such as the Mongols.
It is an instantly recognizable structure which many people are familiar with, though often know little about.  
The Great Wall of China is nearly 6,000 kilometers long. In some places, it is 25 feet tall and ranges from 15 to 30 feet wide.
Many thousands of people were involved in the building of the Wall. From records it appears that 300,000 soldiers and 500,000 common people were involved in constructing the original Great Wall under Emperor Qin.