A great improvement on a great game!!
Certainly, most things remain the same, but this is just an expansion, never expected a major overhaul from it. The gameplay is a lot more challenging than Rome, instead of being on Italy and surround by allies, you now have to face the many barbarians thats invading your lands. And when you fight those "Hordes", a new feature of the game, its inspiring and terrifying at the same time when you see the endless tide of men rising from the horizon. There are also many fun faction to play and they are more balance than Rome. Barbarians are no longer the underdogs that can only build minor city, Franks for example, can build the same level of most building as Roman. Roman Infantry no longer dominates everyone, most faction have some top units that are better than the late legionary.
Overall Barbarian invasion provides a very challenging campaign, some cool new units and a lot of cool new features. It might not totally change the game, but it's certainly worth playing.