The major structure built during the Qin Dynasty to unify the Chinese states is the Great Wall of China. Initially constructed by earlier Chinese states, it was later extended and consolidated under the Qin Emperor Qin Shi Huang around the 3rd century BC. The Great Wall was built primarily to protect against invasions from northern nomadic tribes and to consolidate the empire's authority over conquered territories. Over the centuries, it evolved into a massive fortification system, showcasing the engineering prowess of ancient China. Today, it stands as a symbol of Chinese heritage and unity.