Wall

Walls are used to seal off your settlement and slow down enemy attacks. 

Walls come in various forms. They are scalable by units that do not mount horses and provide a height and range advantage for them. Moreover, units stationed on walls suffer less damage from troops that fire from a lower level (typically on the ground).  Some walls are indestructible my melee units and require siege engines to damage and remove them. Walls also provide access to towers and gatehouses and therefore they are invaluable to site.

Stairs can be placed to provide easy access from the ground or different wall sections for soldiers. Braziers can also be placed on walls, allowing archers (and Arab bowmen in Crusader ) to fire flaming arrows to ignite pitch ditches from range.

Walls are vulnerable on their own if left unguarded, although there are some ways to protect them. Placing traps like killing Pits and pitch ditches can pick off advancing soldiers, but moat can give more time to react and fortify these positions. Stronghold 2 and Stronghold Legends features a number of wall-mounted devices that can be used as a one-shot weapon to clear off units under the walls, such as the rolling logs, the stone tipper and the rock basket.

However, despite these efforts walls are easily overcome by a few methods. The most common way to destroy them, for which siege engines are used, preferrably catapults and trebuchets. Tower mangonels and rams (in Stronghold and Crusader) can also be used to a lesser extent to remove them. If one does not intend to destroy the walls but make use of them, siege towers can be rolled in to provide access from the ground, while assassins may use their grappling hook and climb up on their own.

Different wall types in games

Single Stone Wall: a single tile of wall. Can be drawn in sections and multiple layers can be placed adjacent to each other, effectively creating a thick wall section. They are fairly cheap to erect and they can host braziers. Stronghold 2 and Stronghold Legends enables to create double and triple thick walls for an equally higher price. In Crusader, low stone walls can be placed with half the height and health, but for a halved price.

Wooden Wall: a single tile of wall, created from wood instead of stone. It behaves just the same as the stone wall, but it can be damaged by ground soldiers and withstands fewer hits. It appears in Stronghold and Stronghold 2.

Stairs: they are effectively wall segments with different heights forming stairs. They can connect the ground and the wall, but they do not provide access on their own to towers. Some buildings in later games have their own stairs attached to them. They are substituted by the stairwell and the wooden platform in Stronghold 2.

Crenellation: these walls provide partial defense for units stationed on walls in Crusader. They are placed adjacent to a stone wall segment and their upper sections can absorb more shots because of their height.