The training of Orcish armies takes place in this structure. Grunts, Spearmen, and Raiders all practice the skills needed to slay the enemy in the cruelest form imaginable. When your forces have been depleted and more warriors are needed, fresh recruits can be found here. The crews that man the awesome catapults are also trained here, and when one of the mighty devices is required, it will be routed through here as well.
Barracks are used to train most of your troops, Grunts, Spearmen, Raiders 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).