The Barracks is where you will build your troops. You will come here when you want to upgrade your troops. There are four different types of trooops that you can train.
On the left side of your barracks, when you enter, is where it displays how many troops you have out of what is available. The higher the barracks, the more troops that you can store here. In the middle of the barracks, you will see how much resources you need in order to train your troops. On the right, you will see 5 boxes with stars. These are the level of your troops. You begin the game without any stars, but you can upgrade all the way to 5 stars. Click on the double, blue arrow ups to see what you need to advance your troops to the next level.
When you have enough Spirits, and your Barracks is high enough, you can then click on the double blue arrows. For example, when you click on your Spearman, to advance them to two stars, you see below what you need:
Spearman are unlocked when the game first begins. Archers unlock when your Barracks is at level 10. Dragoons (calvary) unlock when your Barracks reach 20. Junior Warlocks (mages) unlock when your Barracks reaches level 30. The last two troops, Guerrilla, and Wizard, are not unlockable.