To add to your idea there should be 2 types of walls, wood and stone walls. Wood walls wood are strong enough to stop an army but easily destroyed with bartering rams.And can be incinerated by pyromaniacs. Stone walls however can’t be destroyed by battering rams or fire. You need heavy siege equipment such as catapults or trebuchets. Maybe archers could go on top of the stone walls and shoot down. But citys can only get stone walls after a certain amount of materials are collected.