I think a min amount of damage on cities is a good idea. I wouldn't go too far with it, as this would be the min amount. 10 or 15 sounds appropriate to me. It also isn't really necessary. Your main defense should be an army. A city caught without walls or a garrison shouldn't be able to do much to a barb unit. A single barb unit doesn't pose much of a threat either, other than pillaging your tiles, which is their right at that point.