During the Warring States Periods in China (476 BC - 221 BC) many walls were built by the different rulers to defend the regions from the enemies.
The Emperor Qin Shihuang unified the states and ordered to link the different walls and make an extension to prevent an invasion.
The Great Wall was built by common people, soldiers and criminals. Many of them lost their lives due in the construction.
Other dynasties continued extending and fixing the wall.
The longest extension was built during the Han dynasty (206 BC - 220).
The most popular spots of the Great Wall, like Badaling or Simatai, were built during the Ming dynasty (1368 - 1644).