<human>:  Why was the Great Wall of China built?
<bot>: The Great Wall was built to protect China from the nomadic groups that lived in the north. The wall was built from the Yellow River to the sea, and was originally about 20 feet tall. The wall was made of earth, wood, and stone, and it took hundreds of thousands of people to build it. The Great Wall was eventually abandoned, but it was rebuilt in the Ming Dynasty (1368-1644).