As an added strategic element, I think the next version of Civilization should include unit "facing." Basically, when ever you move a unit, you can select the way it "faces", as in the direction that the unit or army is marching. The choices would be North, South, East, or West, to keep it more simple, but the idea is in when you go to attack another unit, cities are NOT included.
As an example, consider a 3x3 grid. The center tile is where your unit/army would be. In this example, your unit is "facing" North. If you attack a unit to the North, North-West, or North-East, who is facing South, it would be considered a head-on attack, and there would be no change in combat calculations. However, if the target unit is facing East or West, this would be considered attacking from the side, and give you a slight advantage, so as to simulate flanking. Finally, if the target unit is facing North as well, you would be attacking them from behind, and this would give you a significant advantage.
Certain unit types could have fun with this idea, as well as special promotions to enhance and defend against this new strategy.
As an example, consider a 3x3 grid. The center tile is where your unit/army would be. In this example, your unit is "facing" North. If you attack a unit to the North, North-West, or North-East, who is facing South, it would be considered a head-on attack, and there would be no change in combat calculations. However, if the target unit is facing East or West, this would be considered attacking from the side, and give you a slight advantage, so as to simulate flanking. Finally, if the target unit is facing North as well, you would be attacking them from behind, and this would give you a significant advantage.
Certain unit types could have fun with this idea, as well as special promotions to enhance and defend against this new strategy.