You start off with a base to defend, whether it's a town, a tower, a castle, whatever. You have to defend against incoming armies, build up your own, and then push back. Grab a command flag, charge at the opponents base, destroy their armies, and take their base as your own.
Done that? Good. Your new base is now the starting point for the next battle. This cycle repeats until the end, unless of course the opposing team captures your base. Then you go back to the previous base and the game turns into a tug-of-war.
Nearly everything in the game can go on without player interaction, so while you are just another unit in an army, you can shift the balance to decide between victory and defeat.
There will be a wide range of characters to control, and plenty of campaigns to play out.
Depending on success, I will either eventually open-source the game, or allow for modding.