While various walls were built as early as the 7th century BC, the first unified Great Wall was commissioned by Qin Shi Huang, the first emperor of a unified China, around 221 BC. The wall was built to protect the fledgling empire against raids by nomadic groups from the Eurasian Steppe, such as the Xiongnu. Constructed using rammed earth, wood, and later stone and brick, the wall also served as a means of border control, allowing the imposition of duties on goods transported along the Silk Road. It functioned as a massive communication corridor, using smoke and fire signals to transmit messages across the frontier. The wall we see today is largely the result of Ming Dynasty reconstruction (1368–1644). It stands as a testament to the immense logistical and human capacity of ancient Chinese dynasties, though it also became a symbol of the suffering of the millions of laborers who died during its construction.