The Great Wall is (1) a wonder of the world. Now, millions of people journey to the Great Wall each year, making its most popular sites (2) besieged by (3) hordes of tourists during (4) busy season. The Chinese have a long history of building walls, (5) dating from the Warring States period. In history, about 20 walls were built, with the wall constructed during the Ming Dynasty being the longest, extending 6,700 kin. China was the most technology-advanced nation in the world then, so the wall was also the most sophisticated in structure. It was built to (6) ward off the invasion of nomads from the north.