Therefore, it is called the city gate, according to its shape, that is, the ancient city has doors on all sides, one side goes in and out in front of the city gate, and there is a pool outside the door, and the gas in the pool also goes in and out in front of the city gate, so it is called the city gate. In addition, it is also called a nozzle. If the land is surrounded by mountains and there is a gap for external air to enter and exit and internal water to enter and exit, this mouth is called in front of the city gate, or Shuikou. There are gaps on both sides of the vertical edge. If it meets the requirements, it can be said to be the city gate.
You should be right to understand this.