The world's longest wall is the Great Wall of China. This ancient structure was built over several dynasties, starting from the 7th century BC, primarily to protect against invasions and raids from nomadic tribes. The total length, including various branches and sections, extends over 13,000 miles (21,196 kilometers). The Great Wall is recognized as a UNESCO World Heritage site and represents an extraordinary feat of engineering and construction. It attracts millions of visitors annually, serving as a symbol of China's historical strength and cultural heritage, while also illustrating the challenges faced in maintaining and preserving such an expansive structure over time.