Barbarian Invasion features several new enhancements to the game, including Religion, Hordes, Emergent Factions, Rebel Factions, and more. Each of which will be detailed further below.
Where RTW featured 103 territories on its Campaign Map, BI only possesses 72. Because of this many factions begin the game with only a single territory. This, along with territories being generally poorer than in RTW, significantly alters gameplay. The player will often have fewer troops than were available in RTW, and each battle will have greater impact. Money will be harder to come by, so the wise management of resources becomes more important as well.
There are 21 factions in the game. 10 are playable from the start. Those listed under Unlockable can be made playable through a simple modification of the game files. The Non-Playable factions require much further modification to be played, as they are Emergent Factions (see below).
* Western Roman Empire
* Eastern Roman Empire
* Sassanid Persia