As you might have guessed, the most significant change in the expansion is the presence of the roaming barbarians. Those hordes will storm through your territory with enormous armies that will easily overwhelm even the most powerful defenses. If you recapture a city that had been taken by barbarians, a new horde will spring up as they become displaced once again, threatening new areas.
Novice players should take up arms as one of the barbarian hordes first. Hordes have no settlements or politics to worry about; just loot and pillage your way across Europe in an ancient preview of Hitler's world tour.