6. Xi’an, China
Dating back more than 3,000 years, Xi’an is one of the oldest cities in China. Its existing walls date back to the 14th century, when they were rebuilt during the Ming Dynasty. The walls around the city are not only one of the best preserved city walls in the world, they are also one of the biggest with 14 km long, 12 meters high and a thickness ranging from 12 to 18 meters