Barbarian Invasion introduces the concept of a 'Horde' faction - one without ownership of cities. In this way, a faction which loses its last city isn't reduced to insignificant rebel bands, but its populace becomes mobile. This is the situation the Huns and Vandals find themselves in at the outset.
When they conquer a city, a portion of their troops will settle, forming the new static population. The rest will settle in the new nation's second and third cities until only the professional military troops remain mobile.
But as long as they remain in Horde mode, there's no upkeep to pay for these armies, resulting in a highly dangerous and unpredictable foe.
This means that the Huns and Vandals possess no income and no safe haven at the outset. They must rely on their large cavalry armies to find a new home. At someone else's expense. It's easy to imagine the terror this inspired in firstly the Goths and Franks, and then the Romans.
Will you settle immediately, and build a power-base in the remote steppes? Or will you pounce upon weak neighbour nations in a blitzkrieg storm of conquest? Or charge straight for the prize: Italy itself?
The odds may seem heavily stacked in the Barbarians' favour, but there are chinks in the easterners' armour. They possess the finest, most effective cavalry yet seen on European battlefields, but their infantry don't match up to the mighty Roman legions.
Just as tactically significant is the very limited siege experience possessed by both Hun and Vandal. Even if Attila and chums had siege towers or sappers, they're hard to integrate into a cavalry army.
At the battlefield level, other new factors will challenge commanders who thought they had cracked Rome's tactical nut. Rivers are no longer an impassable barrier because many units now have the ability to swim.
Battles can also occur after dark, and night-fighting is an ability among a new batch of Generals' traits and retinues that'll add even more character to your leaders.
Religious leadership will also play a more important role in Barbarian Invasion than in Rome itself.
A Pagan leader such as Attila the Hun will naturally be a Pagan, have a Witch or Witch Mother in tow, boast a terrifying personality and be one of the lucky few with a unique weapon which improves his abilities.
Occupying a territory with a different religious base to your faction will cause major unrest. Time to introduce your new subjects to the advantages of conversion to your faith.
You'll have to pay attention because the campaign map has changed too. Borders and cities have altered to reflect the passage of time; overall, there are fewer territories and more space in between.
In addition to the 10 playable factions (Western Roman Empire, Eastern Roman Empire, Huns, Vandals, Saxons, Alemanni, Franks, Sassanids, Goths and Sarmatians), there are plenty of others who may play a significant part.
One of the Roman factions' most pressing problems will be rebellion. Generals with a low Loyalty rating (another new feature) may decide to split from your empire, taking equally disloyal Generals with them and their lands too.
The result will be a new faction: the Eastern or Western Roman Rebels. Some of the smaller tribes which dot the map may also develop into proper factions.
The dramatic changes to the game's factions naturally mean that very few units remain the same. Even Roman troops reflect the passage of time and change in both military and social situation.
Rome itself offered a vast breadth of strategic challenge, except in one area: each faction was essentially tasked with the same goal, of gradual outward expansion.
Barbarian Invasion injects welcome focus to every faction's goals. Wargame commanders have never been so well served or sternly challenged.
Rome Total War Barbarian Invasion will be available for PC in September
