I had an idea that you might like: If you capture an enemy militar building, you can recruit units of that civilization as mercenary units paying with metal and you can recruit only mitilar units that your faction don´t have as mercenaries. For example, if you are using Spartans faction and you capture a iberian barrack from your enemy, you can recruit iberian swordmen from that barrack for maybe... 100 metal. . If you capture a enemy workshop from Romans, then you can recruit a balista as mercenary units for maybe... 300 metal.