2. Great Wall of China
The Great Wall of China, which still holds the title of being the longest wall in the world, is a natural defense line that has been built for centuries. According to the legend, during the completion of the Great Wall of China, 1 million people working for this structure have lost their lives. The building, the ruins of which are 2,500 kilometers long today, is said to have reached 8,851 kilometers when completed. Every year millions of tourists visit the Great Wall of China, which was added to the UNESCO World Heritage List in 1986.