Wall typically refers to the protective fortifications around towns. Building a Fort adds walls around the town, and building a Castle (structure) strengthens them to fortified walls. Town walls block the movement of ground walking creatures and hamper the attacks of ranged attackers. Walls can be damaged and destroyed with Catapults or Earthquake spell. Additionally, cyclopses and cyclops kings have the special ability to attack siege walls.
At the middle of the walls is the drawbridge, which can be damaged and knocked down the same ways walls can. The drawbridge can also be opened by the town defenders, but not by attackers. However, if a stack dies while standing in the open drawbridge, it will not close.