After reading some random article, it sounds like if you build them in cities and have a unit guard/sleep there, they will accumulate the Barracks xp bonus? I originally thought that just by simply constructing one, all of your military units get XP regardless after reading CivAnalyst's description of Barracks. So does this mean each Barracks in each different city provides additional XP if you have a military unit guard/sleep their for a turn or so or is it only one-time? Or maybe I'm completely wrong and just simply building Barracks DOES increase the XP of military units regardless of where they're at?