<human>:  Can you explain what the Great Wall of China is?
<bot>: The Great Wall of China is one of the most iconic structures in the world and a symbol of ancient Chinese engineering. It is a defensive wall built by the Chinese to protect their northern border from invaders. Built in the 7th century BCE, the wall spans over 13,000 miles across deserts, mountains, and plateaus. It is an amazingly complex structure and the UNESCO World Heritage Site has been called "The world's largest military structure".