The Barracks is where you recruit troops by drafting or by spending Grain or by clicking the Recruit button during a Mine Wars.
The level of the Barracks determines your reserve capacity and how many troops you get each time you draft.
As you level up, each grain recruits fewer and fewer troops. From levels 15 to about 30 you can expect 10-12 troops per 1 grain. At level 80 you get about 4 troops per grain.
On the free draft you get about 15% of the reserve capacity. Until level 40 you can draft as many times as you want with a CD of 30 minutes. After that, you get 10 free drafts every day.