Walls are the most basic defense used by all sides during the war, capable of preventing infantry and light vehicles from going through although heavier vehicles can crush them, thus making walls only viable in early stages of the game or to prevent key structures such as labs from being captured or infiltrated.
The Allies use Concrete Walls while the Soviets use Fortress Walls, Epsilon uses Citadel Walls and Foehn uses Bastion Walls; neutral walls also exist. They only differ in appearance and have identical stats with each other.
Concrete Walls are the most basic of defenses, preventing most infantry and some vehicles from entering an area. Cheap and quick to build, they are an Allied commander's last line of defense and can mean the difference when every second counts.
Fortress/Citadel/Bastion Walls are the last line of defense for a Soviet/Foehn commander/Epsilon proselyte. Cheap and quickly built, they serve as an early deterrence to infantry and light vehicles. Essential for protecting vulnerable structures against infiltration or capture.
Walls are basic obstacle for keeping out enemy infantry and vehicles. They are cheap and fast to build, and are the first lines of defence against the enemy's advance. They are often placed around key structures, such as Construction Yards, to keep out infiltrators and engineers. They are also often placed in front of certain base defences to draw fire, as building new wall sections destroyed in an enemy attack is often cheaper than repairing or building new base defences. Note that most base defences may not fire over walls.
The main purpose of walls is to hinder attacking ground (i.e. non-flying) troops, allowing defenses to damage and kill attackers as they attempt to breach the walls. Once breaches occur, the attacking enemy have free rein to destroy the defenses and other buildings within the walls. Therefore, when designing a defense, it is crucial that players do not to attempt to envelop their entire base within one big wall. Instead, it is recommended that players design their walls in layers or cells so that ground troops have to break through multiple walls to attack the entirety of the base, while also providing the defenses more opportunity to repel the attack. Note that with this strategy there will inevitably be buildings that cannot be placed within the walls; that is entirely normal and can even be used to your advantage by placing non-defensive and non-resource buildings such as power plants and technology centers immediately outside the walls to act as another layer for enemy units and tanks to fight through.
Wall sections could not be breached by small arms fire, but are vulnerable to explosive and tank shells. Walls also block all ground units with the exception of omnicrushers. Air units can bypass walls entirely. If there are two layers of walls, the blast radius from area splash damage will affect both layers. Walls will not be targeted by air units, but they can destroy walls through indirect means.
Behind the scenes
Initially Foehn was supposed to have Windbelt, which doubles as a power structure and an impassable barrier, akin to the Comet Fence. However, possibly due to balance issues as the Wallbuster are capable of instantly destroying walls (the Windbelt included), it was replaced by Bastion Walls as static defense and the Windtrap as a power structure.
The remapable Allied and Soviet Walls are public assets and can be used by other modders.