I notice over time you get less from killing barbs to the point where you are at lvl 10 (100xp) and no longer gain any XP. This only seems to change when I declare war on another civ and start trashing their units and cities. Then I can start gaining XP and levels again. Can someone explain this in more detail to me please? I know you get more XP if killing a unit where the odds are quite low to win, like 30% or something. But if you kill a hero or another special unit shouldn't you get some sort of extra reward?