- Cannot be built if Stable has already been built in this district
The Barracks is a great overall booster for your city, providing a wide array of small bonuses. Even better, it's available very early in the game!
Note that you have to choose which one of the two basic buildings you will construct in an Encampment - this one, or the Stable! One aims to boost melee, ranged and anti-cavalry units, while the other one boosts cavalry (and surprisingly, siege units). So, depending on your available resources, and on your particular military style, you should choose wisely.
Militiamen had homes, had jobs, and fed and clothed themselves, waging war only when called out by the government; professional soldiers had no homes, war was their occupation, and they got their victuals and spiffy uniforms from the government. Once writing was invented, professional armies were possible (along with most everything else, including kings and civilizations). Since they didn’t have homes as such, the barracks housed them … usually near the center of town so the soldiers could react to any threat, external or internal. Effectively, the barracks were permanent camps, with such amenities as training grounds, bath rooms, medical facilities, mess halls and gymnasia added. Often these barracks were at least minimally fortified, just in case of trouble. Be it ever so humble, home is where the heart is … in this case, war.