The Great Wall of China is the longest structure made by man. This stone dragon, which it is often called, is more than just one wall. Many different segments make up about 13,000 miles of defenses. These fortresses were built by many different groups of people over 2,000 years, using rammed earth, quarried stone, and kiln-fired bricks. The most recent wall, built during the Ming dynasty, has many interesting architectural features, including guard towers, elaborate fortress passes, and protective barriers. This wall is truly one of the great engineering wonders of the world.