Definitely have Forts n Watchtowers Naough!!!
Turf Wars: Battle of the Outposts
basically, sounds a LOT more interesting than ... City-centric warfare, City-spam, All-or-nothing warfare (tied to city warfare)
The way I see it, people could be fighting rather continuously over certain areas of turf rather than necessarily try to attack the city (a lot harder) to kill the other player.
Indeed, you could have armies that specifically rule the countryside ... basically what I mean is that you would need some kind of Seige weaponry or Wall-destroying magic to even THINK about attacking someone's Walled City, however you don't ever need to attack their walls to cripple them/ win the game.