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.