Fortress Walls (simply called Wall in the game) are the passive defence system for all three sides of the War of the Three Powers.
The Allies use prefabricated concrete barricades originally designed to better organize civilians being evacuated from battlefields and major sporting events. They've proven effective enough to withstand enemy fire as well, so they now surround established Allied bases.
Never ones for subtlety, the Soviets use massive walls of cinder blocks reinforced with steel as their ominous protection for their sprawling bases. Ever paranoid about Spies, each tower has a searchlight in case of night infiltrations.
The Empire of the Rising Sun used a complex nano-alloy to make walls for its bases. Since they're easy to make and relatively inexpensive compared to concrete, the Japanese were free to sculpt them in the shape of the walls that surround Japanese castles.
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 some base defences may not fire over walls.
Wall sections could not be breached by small arms fire, but are vulnerable to explosive and tank shells. Some of the largest armored units such as the Apocalypse Tanks can crush walls (in the case of the Apocalypse, just about anything) with impunity.
If the Proton collider hits a row of walls they will continue to explode until all the walls are destroyed or the row ends.