At some point in the game, raiders become more powerful and can potentially bring rams with them.
In all “let’s play” I saw, they sometimes attack walls.
My suggestion is to be able to prevent this from happening by putting a moat in front of the walls.
There is already the possibility in the game to level terrain, so creating some kind of hole in front of a wall would be a nice addition (a quite time consuming task).
This would redirect raiders to the gates and therefore make the city more robust to attacks.
I’m aware that a watered moat would be difficult to handle due to the uneven terrain, but a dry ditch in front of the wall is effective without worrying about level.