Armour

Armour is the obvious material that helps you defend your fort. It has lots of HP, can deflect some light (=not heavy) projectiles such as machineguns and it is less affected by stress and tension.

Main use
Armour is mainly used to reinforce a fort, as it can tank quite a lot of damage. Indeed, it won't die to most weapons in one shot, except heavy weapons, so you'll need to use several plates in order to be well defended.

Don't use too dense armour though, because splash damage can make your Resources drop extremely fast, since it costs 3.5 metal/second/plate to repair.

Other uses
It is more resistant to stress, meaning it won't break as easily as Bracing when it comes to weight. This has led to foundation reinforcement techniques, proven to be very efficient. In many cases it is more advisable to build additional bracing struts though, especially at ground level where there are mines.

Armour can deflect light projectiles, such as Minigun bullets, if you use slanted armour, you can counter those easily, giving you a huge Resources advantage.

Forts generally are prebuilt with some armour plates, that you can sell in the optic of a rush. They give a bit of extra Metal you can use to build Technology faster, thus having Weapons first.