The Great Wall of China, stretching from the Yellow Sea to the Asian deserts, was built over a period of approximately 1,800 years. Construction was not continual, but waxed and waned in response to barbarian threats from the north. The wall is 25 feet high and 12 feet thick; it runs 1,500 miles across northern China. The purpose of the Great Wall was to make it difficult for raiders to escape with their booty, and thereby discourage invasion. It was not intended to keep invaders out, because it would have been prohibitively expensive to keep it manned.