Idea for a fortress

[Crecer13]

Hello, while watching another lecture on the Middle Ages, I got an idea - why not add a moat to the game for fortresses that are located on the banks of the river? One of the most common locations of cities and fortresses was on the banks of the river (it is obvious why), often in such fortresses a moat was dug from the river, which performed a dual function - strengthening the defense of the fortress and fish (fish were often bred in such moats), in this way the moat can give a bonus to city defense and a bonus to food reduction during a siege. My idea is a slot in the construction as if the city has a river there is available "irrigation"