The construction of the Great Wall of China began in the 7th century BC, with various segments built and rebuilt by different dynasties over the centuries. The initial walls were constructed to defend against invasions from northern nomadic tribes, particularly the Xiongnu. Significant rebuilding and expansion occurred during the Ming Dynasty (1368–1644), where the wall took its most recognizable form. Its purpose extended beyond military defense, symbolizing the strength of the Chinese state and its longevity. The wall also served as a means of controlling immigration and emigration and facilitating trade along the Silk Road.