"Barrack is the place to enlist and employ soldiers. Upgraded barracks can hold more soldiers." Barracks are available at Senatus level 15. After level 15 your troops start dying when you fight and they need to be replenished.
Enlisting gives you free soldiers 10 times a day. There is a 30m cooldown between enlists. The higher the level of your baracks the more soldiers you get per enlist.
Employing trades crops for soldiers. This can be done is you have run out of enlists or in a hurry to get more.