Conversation so far:
where is the the great wall of china located
The Great Wall of China is built across the historical northern borders of China.
how long is the great wall
The Great Wall is 21,196 km (13,171 mi).
why was the great wall built
The Great Wall was built to protect and consolidate territories of Chinese states and empires against various nomadic groups of the steppe and their polities.

Question: when was the wall built
Self-contained rewrite: when was the great wall built