You can recruit soldiers at the Barracks, as well as increase the size of your army. You will then use these soldiers in Boss Clash mode of Monster's Den, which is unlocked at the same time.
In Kingdom Level 35, you will use these soldiers on the Castle and Prison to defend your Kingdom and attack others.
Make sure you have a healthy buffer of available soldiers at all times. This may mean at least 100.000 soldiers until level 35, and at least 200.000 from there. This is fortunately quite cheap, but slow, and best done little by little. Keep recruiting.
Opening the barracks reveals a screen with three areas and four buttons.
The first area is the Strength Chart. It shows which towers win over which, useful information for the Fighting Pits and Prison. Warriors -> Cannoners -> Archers -> Mages -> Warriors
To the right of the Strength Chart is the area for recruiting soldiers. here are:
Your current Available soldiers for battles.
Recruitable soldiers - the maximum number of soldiers you can recruit each time (Each recruitment has a cool down period after use).
Your Barracks Level
There are two buttons in this area, one to upgrade your barracks and other to recruit soldiers. The +/- and > buttons set the numbers of soldiers to recruit. Need Gold shows the gold needed to recruit the selected number of soldiers.
The upgrade screen is fairly self-explanatory, showing the individual recruit limit and deploy maximum for your current level and the next. Below this information it will tell you the cost of that upgrade, and offer you the chance to purchase it, if you have necessary gold and resources (see table below).
You will need it for winning Fighting Pits, Boss Clash and Prison.
NOTE: This is just one of the places when you can increase the size of your army. You can also do it by upgrading the soldiers your heroes command in the Hall of Heroes; but only for the Soldier type corresponding to the Hero selected. It (mostly,) uses Tears of Shiva to do this, it only adds 15 soldiers at a time...and takes a long time, (and a big pile of tears--he he--to make any notable difference.
The number of soldiers you will recruit is determined by the amount of gold you spend on the action, and capped out by the level of your Barracks (see table below). Enter the amount of gold you want to spend first (in practice, use the button for "all you can"), as it defaults to one gold, the minimum amount.
This is relatively cheap. You get around 28 soldiers for each gold spent. However, as you should have a buffer, this can add up. The 200.000 soldiers buffer recommended has a cost around 7000 gold. And depending on your Boss Clash preferences (battling the highest enemies you can defeat causes you to lose more soldiers) and Prison strategies, you may need to replace 100.000 soldiers a day or even more around Kingdom Level 35-40. Take all of this into account. See Prison and do NOT cross the level 35 barrier until you have grown your Kingdom's Battle Power sufficiently to repel attacks from kingdoms 5 Levels above you, (upgrading your Hero's training-equipment, etc. and your Rual Camp and Barracks. Your Barracks is important and should always be the same level as your kingdom level. Do this and Prison should not be much of a problem.
Recruiting is a slow process that must be done repeatedly. Even worse, as you level up your cooldown will not rise the same as than the number of soldiers you can recruit at once. This changes with barracks level (see below), but you will start being able to recruit up to 60.000 soldiers/hour (provided you are always ready to click the button on time), and this will drop to around 40.000 soldiers/hour with medium levels. Only when you reach the maximum cooldown (10 minutes, at Barracks level 38) this will change, and from level 41 onwards you will be able to recruit quicker than at the start of the game. Keep it in mind.
The third section, below the other two, shows each unit type, and the current size of your army. The maximum size has already been determined by barracks level and outfitting the units as mentioned above. This section allows you to reduce the number of soldiers you choose to actually field. This number seems to have no impact on the PvP battle, (I've reduced mine to all zeroes and still won battles) but there is speculation that it can impact the amount of casualties taken in the Boss Clash.
Clarification: Why is there a difference between the number of available soldiers, and the maximum number that you can deploy? During the Boss Clash, you actually lose soldiers during each battle. There is the option to auto-refill your ranks between battles, which is useful as you can opt to run all of your daily allotment of battles (or any number of battles below that max) at one time. This is called a sweep. This auto-refill of the ranks of deployed soldiers takes units from the available soldiers. If there are not enough available soldiers, you fight the remaining battles with fewer and fewer soldiers. It is a good idea to have a healthy buffer of available soldiers.
The two buttons in this area are Max All, and Save Settings. If your army is depleted, or if you have increased the maximum number of soldiers you can field, you can click this button to automatically fill up your ranks, rather than raising or lower these numbers with the individual slides controlled by the red down-arrows below each of the unit types.
(NOTE: You can also drop down the unit types and re-arrange the order of the towers from here if you wish.)
The save settings button is hopefully self-explanatory.
Barracks Level Table
Barracks Level determines the maximum number of soldiers you can bring in Fighting Pits, Prison and Boss Clash, and how many you can recruit at once. You should always have a healthy buffer of soldiers so you don't run out of them, because recruiting is slow.