Why do all the ancient cities in China have moats?

The moat, as its name implies, is used to protect the city. There were no such strong cannons in ancient times, and the average city wall was ridiculously thick. As far as the wall as thick as Daliangmen in Kaifeng is concerned, looking at it really feels a sense of despair. Coupled with the deep and wide moat, it is impossible to fight at all, so it is the most difficult and also the most difficult for the general to cut the plan, then cut the soldiers, and then besieged the city. This is Sun Tzu's art of war.

First of all, the city wall is very high, and the wall of Daliangmen is 26.3 meters high. For example, some big cities, once Nanjing and Beijing, must be higher and deeper. How to attack the city, one is the remote crushing trebuchet. Of course, it is often difficult to hit the city gate, because the city wall is very thick, and many trebuchets go out not only to hit the city wall or the soldiers on it, but also to hit the city.

Otherwise, it is to break down the city gate, as is often shown on TV. Under the cover of a group of people with shields, another group of people carried a long pillar and stormed the city gate. However, some big cities in ancient times had suspension bridges. There is a moat before the suspension bridge rises, so it is impossible to get close to it and swim with such a heavy thing. I'm afraid I'll sink when I enter the water, and the water in big cities has been dug deep.

And a ladder. Get a particularly high ladder to climb the city wall, climb up and kill the soldiers guarding the city, open the city gate and plant your own flag, which basically declares that the city has won, because the most difficult thing is to attack the city, and it is not good without a water ladder. You can't swim with it. Even if the ladder is very high, you can cross the moat. Who has the lightness skill to run on the ladder and shoot arrows all the time? And even if you hit the corner, there will be hot oil and stones falling down.

Therefore, for some big cities, storming is often not the first choice, because storming the ideal city is often three times that of the enemy before you can try it, and you may not succeed. If you win, you will kill the enemy by one thousand and lose by eight hundred. Therefore, under the defense system of high walls and deep trenches, most of them are besieged cities, where food and grass are not available, and people eat people inside until they surrender.