The Great Wall of China is an ancient wall in China. The wall is made of cement, rocks, bricks, and dirt. It was finished in 1878 and it was meant to protect the north of the empire of China from enemy attacks. It is the longest structure humans have ever built. It is about 21,196 kilometers long, wide and 20 metres high. The earlier sections on the wall are made of compacted dirt and stone. Later in the Ming Dynasty they used bricks. There are 7,000 watch towers, block houses for soldiers and beacons to send smoke signals.