The Great Wall of China is one of the most recognizable landmarks in the world.
It is the world's longest structure. In fact, some astronauts have said they could see it from space!
Curving across the land like a sleeping dragon, it follows almost the entire northern border of China for more than twenty thousand kilometers.
You might ask why a nation would need a wall along its border.
The Chinese people built the Great Wall to protect their civilization from attacking tribes in the north.
But the Great Wall of China took over two thousand years to complete, and during this time an incredible amount of history unfolded.
The Qin dynasty was a short-lived but important dynasty because its first ruler, Emperor Qin Shi Huang, was a very influential emperor.
He unified China and started the construction of the Great Wall of China.
He used all his authority to get the wall built.
The builders used the hang-tu method of construction.
They erected two temporary barriers parallel to each other and packed a layer of dirt, stones, and wood between them.
After many layers were built up, the temporary barriers were removed, leaving a tall, thick earthen wall.
Watchtowers were constructed along the wall so that soldiers could see enemy armies approaching.
If invaders attacked, the soldiers lit a fire, and the smoke was a signal to warn the soldiers in the other watchtowers.
Back then there were no mechanical tools to help with construction, so all the work was done by hand.
It was back-breaking labor, and life was very hard for the workers, especially because Emperor Qin was ruthless.
It is believed that hundreds of thousands of men died building it.
The Qin dynasty did not last very long after the death of Emperor Qin Shi Huang in 210 BC.
According to some accounts, he was able to build over 2,500 kilometers of the Great Wall.
After the fall of the Qin dynasty, a leader named Liu Bang came to power and established the Han dynasty.
By the time Emperor Wu Di, the seventh emperor of the Han dynasty, came to power in 141 BC, the Great Wall had deteriorated and needed major repairs.
Wu Di wanted to rebuild and extend the wall.
His workers also used the hang-tu method of construction,
so the architecture of the wall changed very little from that of the section built during the Qin dynasty.
Wu Di's workers, however, had better working conditions, and got paid for their service.
During Wu Di's reign, another large section of the Great Wall was completed.
It was also during the reign of Wu Di that China started trading its riches with countries in the west.
Chinese traders traveled to Central Asia, the Middle East, and even as far as Rome, bringing silk, paper, and spices to Europe.
This long trading route was called the Silk Road, and the main route of the Silk Road closely followed the curves of the Great Wall.
Life in villages along the wall was prosperous and peaceful during this time, but this era eventually came to an end.
After the death of Emperor Wu Di in 87 BC, the Han dynasty slowly deteriorated and finally ended in 220 AD.
It wasn't until a thousand years later, during the Ming dynasty, that the Great Wall became an impressive structure once again.
What we see of the Great Wall today was built during the Ming dynasty.
The first emperor and his successors maintained the Great Wall very carefully, and strengthened it to keep invaders out.
The wall became an important project for each of the Ming emperors.
By the time of this dynasty, new inventions had become available to help the workers with construction.
Workers still used the hang-tu method of construction,
but this time they packed the dirt and stones between two solid brick walls to make it more stable.
They made the top of the wall wide enough for horse carriages to travel, and built more watchtowers.
Now, instead of smoke signals, soldiers used gunpowder rockets to warn of an approaching enemy.
Another improvement made to the wall during the Ming dynasty was a drainage system.
Until then rainwater slowly collected in the wall, making it deteriorate more quickly.
After the last Ming emperor died in 1620, construction on the wall ceased. The era had ended.
For the next four centuries, China was ruled by the Qing dynasty, which did not need the wall to protect its borders.
By the time the Qing dynasty collapsed and the Communist Party took control in 1911, the Great Wall was nearly forgotten.
But in 1980 the Chinese government decided to restore the Great Wall and show it to the world.
Today it is one of the most visited tourist attractions on earth, a silent witness to the many battles, wars, and changing empires of China's history.