Here was my wall model:
With Construction, your troops can build border walls. These walls line one side of a square(like rivers). It take a unit an entire turn to cross a wall if they are not yours. This means that they must start on the square the wall border and it takes the turn to move to the other side. That would be useful for slowing down attackers or preventing raiding. Also, walls coudl be attacked and destroyed. Forst built al;ong the wall would automatically link up and have additonal powers. Great wall would be when a wall walls in 90% of your territory. It would be a small wonder.