It's a classic. You choose to play as any one of the three Roman houses, and you're given orders from the Senate to attack certain places or reach diplomatic agreements with other factions, and the aim is simply to expand the empire. You play on a map of Europe, with all of the factions that existed there at the time, and when it comes to battle you zoom in to the battle map that you've seen on Time Commanders.
It's a turn-based game, and each turn you can choose to build new buildings or train new units in each of your cities, and you can move your armies a certain distance each turn. There's also quite a complex financial system, by which you earn or lose money each turn depending on your spending, the efficiency of your trade routes, the resources available in your land, your trade agreements with other factions, and various other factors.
When you defeat a faction (i.e. completely wipe them off the map), you may unlock them as a playable faction. The unlockable factions are: Greece, Egypt, Seleucid Empire, Carthage, Gaul, Germania, Britannia and Parthia.
Rome 2, which comes out in September, will be have far more advanced and realistic battles and will also have proper naval battles, which the original doesn't really have. You can get ships and use them as transport, but battles between ships are just simulated on the world map and you don't have any control over them.
