The Great Wall is not one wall. It is many walls, joined and rebuilt across more than 2,000 years by different rulers. The most famous parts that visitors see today were built about 500 years ago, during the Ming dynasty, using huge grey bricks and stone.
People often think the Wall is one smooth ribbon, but really it climbs over the tops of mountains and dips down into valleys. Walking it would mean climbing thousands of stone steps. Some sections are so steep you almost have to use your hands.
Building it was an incredible job. Workers carried each brick up the mountains by hand or with the help of donkeys. There were no diggers, no cranes and no trucks. Every brick was lifted into place by people.
Watchtowers sit along the Wall, roughly every few hundred metres. People used to light fires in them at night so messages could be passed quickly from one end of the Wall to the other - a kind of slow-motion text message, made of smoke and flame.