This building allows you to muster armies by training units. The building levels decrease the Training Time for new Units. In the Barracks you can produce your Units and have an overview of your available Squads. Squads are what you will place on the battlefield. On the battlefield, Units of one type are grouped into Squads.
You are able to build the Barracks after unlocking it in the Research Tree and once you have the corresponding resources. It requires a street connection to the Main Hall, it cannot be sold nor rebuilt, but you can upgrade it.
Each upgrade of the Barracks will reduce the Training Time. It can also increase the amount of available training Slots: You will get an additional Slot every time your barracks increase in size.
Training your Units will cost you Supplies and time. The more Units are trained in a training slot at one time, the more resources are needed to start training and the longer that training will take to complete. Of course you can always finish the training instantly by paying Premium.
Depending on the level of your Barracks, you will be able to use up to 5 slots of training at a time. Meaning you can train your troops using a queue system and the units will become available by slot, when the corresponding training time is over. You can cancel queued trainings in case you change your mind and in this case you will receive your Supplies back. In order to add the trained units to the Army Camp you then need to collect them by clicking on the army icon on top of the Barracks. Note that the icon will show whenever the first slot is finished, so that you know you now have a new available slot for a new training while the next slot will continue training.
At the beginning, the base Training Size of a Slot in the Barracks is 12, which means that the Slot can only train Units with a total weight of up to 12. Different Unit types have different weights, so some Units can be trained in larger numbers than others. For example: The weight of a “Axe Barbarian” is 1, the weight of a "Paladin” is 6. So, one Unit Slot can train 12 “Axe Barbarian” or just 2 “Paladin”. If you want to increase the Basic Training Size of a Slot you can build one or more Armories.
Here you can see how many Squads per Unit type are ready for the battle.
Squad: On the battlefield you will place Squads, not single Units. The Basic Squad Size on the battlefield is 12. As in the training size, the weight of a unit is taken into account. E.G., with 2 "Paladin" (weight = 6), you already have one full Squad.
“Squad Size Upgrade”: In the Research Tree you can unlock several “Squad Size Upgrade”, each of which will increase your Squad Size on the battlefield by 3 times the number of that technology. Remember the Training Slot size will not increase with this technology. Build and upgrade Armories to ensure that your Training Slots can keep up with filling the Squad sizes.