Cities will now defend themselves with one unit able to garrison and lend a hand. City upgrades will improve defense and add different properties. City walls, for example, will not only grant a defensive bonus, but also allow your city to make a ranged attack. This frees other units up to roam the countryside and get tactical. No longer are your armies relegated to piling into your capital to repel invaders.

Fans of the religion system from Civ IV: you might want to sit down. Your beloved system is gone. The developers found that it added too much predictability to the diplomacy system. For now, the only system Firaxis is talking about to replace religion are city-states.

In Civ V, in addition to the ever-present roaming barbarians, will be city-states: neutral factions that are limited to a single city. If you befriend one, they might grant you a bonus every now and again; militaristic city-states might offer up a combat unit, for example. If you want an extra city, you can still attack and conquer city-states, but they might just be friendly with that full-on civilization you're eyeing for domination. You could find yourself in a war for your actions.

This all ties in to the new diplomacy system and improved computer AI. Firaxis hopes that the AI will play like humans. The AI are a bit more aware of what's going on this time around. Don't expect a warm welcome from Bismarck if he's spotted you massing units on his border; he's going to call you on this. Finally, the conditions for a domination victory have been stream-lined, requiring you to control only the capital cities of every civ, which should help make these types of wins more feasible.