The expansion Barbarian Invasion brings some new features to R:TW. But there's not much in the expansion pack.
The new features in the expansion pack are small, but good. You can now fight night battles with soldiers having torches in their hands lighting up the night sky. If you are a barbarian tribe you can become a horde where you're on the move looking for a new home to settle in. The civilians that moved out are horde soldiers and if most of them die you new city might not replenish its normal population. Last, but not least, religion is a major concept in the game.
The bad is that you still can't fight naval battles and the results are still confusing. The game doesn't really include much historical battles, just 2 only.
It's a solid expansion pack and a great collection to your RTS games.