This is where the men of arms come for training and recruitment. Footmen, Knights, and Archers can be found here, constantly honing their skills as they wait to be called forth into battle. When you need more fighting men, you will come here to get them. The crews that man catapults are also trained here, and when you require the services of one of these great machines, it will be routed through this site.
Barracks are used to train most of your troops, Footmen, Knights, Archers and Catapults are trained here. Build one and more as soon as resources allow it; the more, the merrier. The bulk of your army will come out of this building, so protect it when under attack.
Upgrade Shield Strength
Increases the armor of all your units and buildings by 2.
The computer player does not need resources to build units.  It does, however, require the prerequisite buildings (for example, it cannot build Catapults if it lacks a Blacksmith, or if you have destroyed it).