The Great Wall of China was not built all at one time. In 7th and 8th century BC, battles happened frequently among the states of the Spring-Autumn and Warring States dynasty and in order to defend themselves they began to built walls and towers on the borders. It was the state Chu who first built the wall. It was during the Qin Dynasty that the kingdom of Qin united the different parts into one empire. To defend off the invasions from northern invaders, Emperor Qin Shi Huang had all the walls joined up. Thus, the Great Wall came into being.
Since then, the Great Wall was rebuilt, modified or extended throughout Chinese history for over 2,000 years by millions of Chinese people drafted in for the task. The primary purpose was always to protect the Chinese Empire from the Mongolians and other invaders. Most of the current Great Wall we see today was built in the Ming Dynasty (1368-1644) and is approximately 6000km long.
The Great Wall of China was built to protect China from its enemies and invaders from the North, especially the Mongols. The Mongols were a tribal group that would regularly conduct raids into China. Despite the wall, the Mongols eventually conquered China. The Wall also kept Chinese citizens from leaving China.
Actually, China is not the only country in history that built wall along its boundary. Athens, the Roman Empire, Denmark and Korea all did so at certain time in the past. The Hadria Wall in northern England, built “to separate the Romans from the barbarians”, extended 117 kilometres from Wallsend-on-Tyne in the east to Bowness-on-Solway in the west. All the walls were built for the purpose of military defense, and the Great Wall of China was no exception.
