The Great Wall of China is one of the most impressive feats of building on Earth. Like the previous two walls on this list, it is also a World Heritage Site.
It is difficult to know exactly how long it is, but it is believed that it was once over 20,000km long. Just as Hadrian's Wall created a barrier between the Romans and northern tribes, the Great Wall of China was erected to create a barrier between the civilised world and apparently less civilised tribes beyond.
It started to be built as early as 481 BC and is more than seven metres high in most places. But it is the Ming Dynasty (between 1368 and 1644) which is credited for the incredible landmark as we know it today. You can see where different sections have been built over the years, with some stretches made from stone, and others from earth and wood. The Great Wall of China stretches from the Yellow Sea in the east near to Beijing, to the Gobi Desert, making it the world's longest man-made monument. There is a rumour that it is so big that you can see it from space, but this isn't actually true. Nowadays, about one-third of the wall has disappeared due to, for example, erosion or people taking the building materials. Drones are now being used to map out the wall and analyse weak points or areas that need restoring.
(from: https://www.bbc.co.uk/newsround/48279965)
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The Great Wall of China: Perhaps the greatest collection of walls in human history.
The Great Wall of China was not always so great. It began as a series of independently constructed walls as long ago as the 7th century BCE. Over time, they became connected into a single vast wall spanning a staggering 5,500 miles during the Ming Era. The wall’s militaristic purpose was to defend China against the multitude of invaders that plagued the borderlands – primarily the Mongols and the Manchu. But the walls provided other economic and social benefits for China, allowing the Chinese to enforce economic duties along the Silk Road as well as decrease the number of immigrants from Central Asia. As a means of maintaining control over citizens of China and their trade, the Great Wall was quite successful. However, the wall failed to keep invaders out entirely. Genghis Khan and his Mongol warriors, the Liao, the Jin, and the Manchus all managed to invade and take territory across the wall. Though this wall has never been altogether destroyed, its maintenance was, and is, such a colossal undertaking that large sections of it have fallen into disrepair over time. (from: https://origins.osu.edu/connecting-history/top-ten-origins-walls?language_content_entity=en)