長城, or the Great Wall of China, is a series of fortifications built to protect Chinese states from invasions. Construction began as early as the 7th century BC and continued for centuries, with the most famous sections built during the Ming Dynasty. The wall stretches over 13,000 miles across northern China. The Great Wall is not a single continuous structure but rather a collection of walls and trenches made from various materials, including earth, wood, bricks, and stone. It is a UNESCO World Heritage Site and symbolizes China's historical strength and architectural ingenuity.