Age of Charlemagne expands Total War: Attila with a new campaign setting that takes it into the Medieval era. The first big Attila expansion—The Last Roman—laid the groundwork for this. That was set a hundred years after the fall of the Western Empire, and focused on the fascinating human story of Belisarius, the overly-loyal Roman general—probably the only man who could have revived it. It featured a really interesting mobile Roman faction, The Expedition, and four less interesting rejigs of barbarian factions that had survived to that era.
Age of Charlemagne takes that structure and runs forward to the start of the medieval era, with a much more limited map that barely encompasses the Mediterranean. What Age of Charlemagne does offer that The Last Roman didn’t is a better range of factions.
The worst of the factions are the Lombards—German invaders who’ve displaced the Romans from the north of Italy. They’ve got a buffed version of the tributary-state mechanics of the Sassanids from the main game, and little else to recommend them. Similarly, the The Emirate of Cordoba in Southern Spain, has a different (worse) take on the tributary state mechanism, fast research and reduced religious unrest. Then there’s the pagan Saxons from Westphalia, who get bonuses to raiding, focused on the Frankish kingdoms, but are a bit surrounded and doomed. All of these factions have standard dull units, with few advanced troops—this is doubly sad for Cordoba because the history of the Umayyad Caliphate is fascinating.