Wall refers to the protective fortifications around towns in a siege. Building a Fort adds walls around the town, and then a Castle strengthens them to fortified walls. Building a Citadel has no effect on walls - it only adds main Arrow Tower and Moat to fortifications. Town walls block the movement of ground walking creatures and hamper the attacks of ranged attackers by 50%. Walls can be damaged and destroyed with Catapults, Cannons (with HotA) or Earthquake spell. Additionally, cyclopses and cyclops kings have the special ability to attack walls during a siege.
On the battlefield walls fill up 11 hexes, and 6 of them are indestructible. Wall hexes are divided into:
W hexes: 6 indestructible segments
(1) hex: Upper Wall
(2) hex: Mid-Upper Wall
(3) hex: Mid-Lower Wall
(4) hex: Lower Wall
(D) hex: Drawbridge
Moat (if Citadel or Castle are built in the town) occupies 11 hexes (or 22 hexes in Fortress) to the left of the wall.
Destructible wall segments have 2 hitpoints (3 with Castle built), except Drawbridge which always has 2 hitpoints. During a siege hero forces can move onto destructible wall segments after destroying them.
The drawbridge is the main pathway into a castle walls during siege combat. The drawbridge is closed by default. The drawbridge can be opened by the town defenders, but not by attackers. Creatures crossing it do not suffer moat damage. It opens automatically when defender moves his stack out of the castle through the (D) or B hex. If the drawbridge is lowered it additionally occupies the B hex on the battlefield map. The drawbridge automatically closes when there are no creatures on the (D) and B hexes. The drawbridge remains permanently lowered if it is destroyed or if there is a slain creature on at least one of the hexes. The latter is called "drawbridge trapping" and can often be utilized against computer opponents sending melee creatures into combat.
If the drawbridge is closed attacker cannot move into the B hex. But if the attacker occupies hex marked as x on the illustration above, the drawbridge cannot open and the defender cannot move out of the castle. The same applies if Force field spell was cast on the x hex or if there is a slain creature on it. Note however than although it is possible to cast force field on the B hex it won't prevent the drawbridge from opening.
The drawbridge has always 2 hitpoints but it's usually more resilient than other wall segments, because it's more difficult to intentionally target it (compare Ballistics).
A legion of Wolf Raiders standing on the open drawbridge