Conversation so far:
How long is the Great Wall of China in miles
An archaeological survey carried out in 2012 by China’s State Administration of Cultural Heritage suggested the wall is some 13,000 miles – or 21,000km – long.
Where is the location of the Great Wall of China
The Great Wall of China is an ancient series of walls and fortifications located in northern China.
How long ago was the Great Wall of China built
The Great Wall of China was built around built around 500 years ago.

Question: Why was the structure built
Self-contained rewrite: Why was the Great Wall of China built