Rome: Total War kicks serious ass. It's Civilization and Warcraft combined, but with nicer graphics and easier control.
It is also much more difficult than medieval.
Really? This is the first Total War game I've played, but I'm loving it. Just started the Imperial Campaign the other night (I was having serious computer problems that prevented me from making it through the Prologue—I ended up playing the first half about 10 times before I gave up) and ended up losing track of about two hours after I'd already been playing for several. Early in the campaign, playing as Brutii, I have 5 cities, am ranked 3rd, and will soon wipe out the Macedonians.
Brutii is harder later and easier early. If you want a quick taste of difficulty, try Scipio.