In a long Civ4 war (on Prince) against Shaka, and I've defeated about five big stacks of units but they keep coming, every two or three turns more units than I could ever build in such a short time. He won't settle for peace and sooner or later however I replay, I get overwhelmed. He's on Hereditary, Vassalage and Caste - how does the AI churn out such huge stacks so quick?