Kalayn Walls are an evolution of standard fortress walls. They were invented by Aboriginal engineers during the dawn of high velocity, rifled, breech loaded, high accuracy, long range, explosive artillery in the late 19th century. Kalayn walls are usually constructed at an angle to protect from indirect fire, like that of howitzers. They are a simple layered mixture that can be repeated or extended for further protection. -The first layer is brick, rocks, concrete, reinforced concrete or other common construction materials. These protect against small arms fire and small calibre attacks. -The second is a very wide layer of dirt, mud, sand or clay. This one catches and detonates bigger projectiles with very little damage to the structure. -The third layer is a very thick steel plate meant to block the biggest, fastest and heaviest of projectiles. -The fourth layer is a solid wall of reinforced concrete meant to work as a last stand. These can be as thick or thin as needed. Smaller fortifications may be able to block tank fire, while huge fortresses like those in Sydney require a superheavy artillery piece to crack open.
However, despite their incredible resilience, kalayn walls are as vulnerable to bunker buster and heavy airdropped bombs as any other static fortification, and helpless against nuclear weapons.