Beginning in the Cold War Age, every Barracks upgraded to level 10 or beyond provides expertise to your troops. This grants a permanent boost to their damage and hitpoints. Each Barracks (level 10 or higher) adds to the overall Expertise bonus.

All of your Barracks troops benefit from the increased damage and hitpoints from Expertise - regardless of the troop’s level.

For example:

You have a Cold War Age base with 3 Barracks. Two of these Barracks are level 9, and one is level 10 and grants Expertise. Even if you train a Cannon (Enlightenment Age unit) at your level 9 Barrack, it still is given the bonus to damage and hitpoints.

Yes! Troop upgrades through the Armory is the only way to advance your troops through the ages. These upgrades also increase that unit’s base damage and hitpoints.

The Expertise bonus is added in after all other troop bonuses are calculated. It does not get boosted by other bonuses.

Does Expertise from my Barracks affect defensive units?
It does not. Expertise only benefits your offensive troops trained from the Barracks.