Medieval II: Total War fills the gaps of Rome: Total War but has it's own flavour!
I'll split my review in two parts, the very thing the game does.
The campaign.
Where the R:TW AI stopped, the M2: TW goes on. The M2: TW AI is way better than Rome's and after playing Empire: Total War, I also made the conclusion that it's better than the newest TW-game.
Gunpowder units may not be the best units in this game, but it certainly adds something new to the game. Rome: Total War had the political and military change when Marius came; converting all Roman units to better ones (except the triarii, but that's just me). Medieval 2 has the founding of a new world (America), the discovery of gunpowder, the Mongolian hordes and the Timurids. It's city/castle management is exactly what was missing in R: TW. There is absolutely a feeling of satisfaction when conquering a vast citadel which was occupied by some French people!
Managing cities hasn't changed much, though. There are only a couple of things where you must pay attention for. 1. Religion. Keeping your people catholic or muslim is a a good preventer of rebellion. 2. Heretics and witches. Killing or denounce them, helps improving religion. 3. but not least: enemy armies (of course).
If you want some diversity in the campaign, I suggest getting M2: TW Kingdoms. There are practically no changes in the battles, except for the units but if you like the campaign, I really recommend to buy this expansion.
The battles.
The battles haven't changed much when it comes to the gameplay. However, there are some additions to the original. The 'sappings'-siege machine is gone. To the contrary, there are more aand better siege machines when it comes to cannons and such.
Towers only shoot at you, when manned. So sending a single unit to the rear, is much safer.
The AI-morale system is, I think, better than Rome. And there are more actions between the soldiers when they're fighting.
This concludes my review. I hope it made you buying M2: TW, because it is an awesome game!