 Tong Pass was built in 196 AD by the warlord Cao Cao during the late Han dynasty.The fortress was the seat of Tongguan County, but was demolished in the 1950s to make way for the Sanmenxia Dam and reservoir.Chinese civilization first grew up along the Wei, Luo, and Yellow River valleys of the Loess Plateau before expanding out into the "barbarians" who held the North China Plain.