The Great Wall of China has its origins in the Warring States Period, which was the last era of Chinese history before Emperor Huangdi began the first imperial dynasty. The states of Zhongshan, Qu, Wei, Qi, Yan, and Zhao each built walls to protect themselves against the others, using techniques developed in the earlier Spring and Autumn Period.

These rudimentary fortifications were constructed by packing earth between wooden boards. When Huangdi unified the states into the Qin Dynasty in 221 B.C., he called for all of the walls to be dismantled apart from the ones that protected the northern reaches of the empire from enemy nomads from the Xiongnu tribe. These early incarnations of the Wall have mostly succumbed to the erosion of time.

The Han Dynasty replaced the Qin in 206 B.C. and the Wall was forgotten until 130 B.C., when the Wudi Emperor recognized the importance of defending the state against Mongol invaders. The Wall was extended westwards through Gansu Province and into Xinjiang, down the Hexi Corridor that was part of the Silk Route. During the Three Kingdoms era that followed the Han Dynasty, only the Kingdom of Wei was concerned with maintaining the Wall, since they needed to defend against the Rouran and Qidan tribes.

When the Tang Dynasty rose in A.D. 618, the Great Wall was again abandoned and fell out of use until the simultaneous Liao and Song Dynasties that followed. The Liao controlled the north of China and used the wall to keep out marauding tribes. Unfortunately, their efforts were in vain and the Mongol-led Yuan Dynasty established itself in 1276.

The Wall fell into dereliction once again until the Ming took power in 1368. In order to keep out the Manchus and Mongols once and for all, the Hongwu Emperor pushed for a comprehensive renovation and extension of the Wall. The most famous sections were built between 1569 and 1583. Since they were built from brick and stone instead of earth and packed sand, they are also some of the best preserved. The Wall was effective until the start of the 17th century when attacks from Manchu tribes began once again.

The Chinese empire held fast until 1644 when a treacherous guard named Wu Sangui allowed the Manchus to enter the country at heavily fortified Shanghaiguan Pass, in protest against the Shun peasant uprising that was threatening Ming power from Xi’an. Wu’s actions cemented the fall of the Ming and the rise of the Manchu Qing Dynasty—China’s last dynasty. During the Qing era, the Wall was partially dismantled for other building projects and fell into disrepair once again.

The Communist Cultural Revolution saw some damage to the Wall, but Chairman Mao was a fan, allowing it to open publically in 1957 at Badaling. It wasn’t until the 1980s that tourists began to add the Wall to their itineraries, following Deng Xiaoping’s restoration efforts of 1984. The Great Wall was declared a UNESCO World Cultural Heritage Site in 1987 and is now one of China’s most popular and iconic sites.

From the appearance of the Wall close to Beijing, it would be easy to imagine that the structure is equally well-preserved all along its length, but sadly this isn’t the case. The watchtowers and passes in Hebei province and the Beijing municipality have been restored to attract tourists, but much of the Wall’s length is derelict. In Gansu Province, vast sections are at risk of disappearing altogether, with just two meters (6.5 ft.) remaining of the original five (16 ft.).