Wall
Wall is term, that typically refers 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, cyclopes have speacial ability to attack siege walls.
A part of the walls is the drawbridge, which can be damaged and knocked down the same ways walls can. Drawbridge can can be opened by the town defenders, but by attackers.