Barracks

Barracks are used to train units. The more barracks you have in your country, the greater the recruit rate is.

Barracks also provide a residential max (as opposed to food max) increase of 20 per barracks (vs 10 for wilderness).

In addition to that, every barracks provides support for 150 recruitable units (ie if you had 100 barracks, the first 15000 archers/cavalry/whatever would not count against your maximum population in the "food and space for units" category)