The barracks gets promotions for each category separately. And it can't get drill for melee Specifically the barracks can gain promotions for melee, archery and gunpowder units (you can edit this in the XML) - separately for each category. Other buildings might only gain promotions for a single category type. The available promotions depend on the category, your techs, traits, their prereq promotions etc. For example: You can give your archery units drill I, and only then you can give them drill II, but if you have a protective leader - you can immediately give Drill II.