Industrial, artillery and factories come out. What I love doing is having a "builder game" on regent, build about 30 cities, get factories and coal plants in each one, have your golden age (with either Russia, Ottomans, Korea, Germany) then mobilize for war. Every city, even if in just plains without river, or any food or shield bonuses, very far from the capital can produce an infantry, cavalry, artillery, or tank every turn. 30 cities X 20 turns in golden age = 600 units. I think world conquest just might be insight.
Industrial is also the golden age for builders, like me.