In fact, Hangu Pass can play a certain role in the past, because that place is the only land without water, surrounded by the rolling Yellow River. If you want to reach the opposite country, you must enter such a place. Moreover, this place is also a treasure trove of geomantic omen, which is easy to defend but difficult to attack, so it was also used as their Xiongguan fortress during the Qin and Han Dynasties. But as time goes on, the water level of the Yellow River drops, which means there will be better places for them to pass.
After all, when the water level of the Yellow River drops, the water depth around it is no longer so deep, and even there is a floodplain next to it, so the enemy can go across the floodplain. However, this behavior only happens when the weather is dry. When the weather is wet and rainy this season, Hangu Pass is still an important Xiongguan fortress. In the following period, during the reign of the Tang Dynasty, the drought became more and more serious, and the place of river beach would appear at any time.
It is precisely because this place will appear at any time that the enemy can attack the opposite side at any time, regardless of time and season. Moreover, Tongguan had been established at that time, and the importance of this place was far greater than that of Hangu Pass, so Hangu Pass became less and less important in the Tang and Song Dynasties, and finally lost its role.