In terms of gameplay, Civ IV is the biggest departure thus far from its predecessors. Firaxis emphasized that they were making Civilization again from the ground up, rather than just taking Civ III and pushing out in new directions. Civ veterans will note that units' attack and defense scores have been replaced by a single 'strength' rating. Each unit then has various bonuses and special abilities - spearmen are better against mounted units, archers get a 'first strike', et cetera. What's more, as units fight and gain experience, they can be promoted, giving them extra individual bonuses (you can, for instance, have a particular unit which is good at guarding cities, or one geared towards fighting in jungle).