The Barracks trains units for your squads and promotes existing units to higher levels. The first Barracks requires Base level 7. Additional Barracks provide more training queues.
How training works
Training capacity is the number of units you can train in a batch. It starts at 30 at Barracks level 1, reaches 370 at level 10 and rises to 570 at level 30. From level 10 onward, each building level adds 10 to the batch size.
Training speed determines how quickly that batch finishes. A larger batch and a faster training timer are separate improvements. Another Barracks lets you use another training queue rather than merely increasing the size of the first queue's batch.
Unit levels and promotion
Higher-level units have Barracks requirements. Promotion lets you raise existing units once the required Barracks level is reached; the game also checks that you have units available to promote. Tier 10 units have a separate technology requirement.
Finished units need room in the Drill Grounds. That is the base's unit-capacity limit. A Squad slot controls how many marches you can send, so more Barracks capacity does not by itself give you another march.
See Troop Training, Promotion and Healing Explained for how these systems work together.
