Although all 3 games require these tasks in different forms, Zeus is the easiest to learn and understand yet maintains a game with plenty of subtle depth. The interface is much more simplistic, and a lot less unwieldy than both games, Pharoah in particular. You once again have goals to achieve, but unlike Caesar, you don't need to please some outside authority figure. At some points in playing Zeus, you can return to a city you were previously involved in. Caesar 3 fans who would build up a city to a beautiful little Utopia, and then have to abandon it, will love this.
Foreign relations enter the game here.
After I look back at the different styles of all 3 games, you can see that the culture of each civilization is seen in the gameplay itself. The growth of an empire, as well as the omnipotence of Caesar, affects the gameplay. During Caesar3, you travel across the growing empire building new cities. THe mission goals for each city clearly reflect a growing empire. If his demands are not met, then you will be replaced by a more competent ruler.