居庸关
The Qin dynasty built a highly centralized state and divided the country into 48 commanderies ("jun"), two of which are located in present-day Beijing. The City of Ji became the seat of Guangyang Commandery (广阳郡/廣陽郡). To the north, in present-day Miyun County, was Yuyang Commandery. The Qin removed defensive barriers dividing the Warring States, including the southern wall of the Yan, which separated the Beijing Plain from the Central Plain, and built a national roadway network. Ji served as the junction for the roads connecting the Central Plain with Mongolia and Manchuria. The First Emperor visited Ji in 215 BC and, to protect the frontier from the Xiongnu, had the Great Wall built in Yuyang Commandery and fortified Juyong Pass.