if u wana talk about the scale of civ, then y include a atvatage for atackers if its gonna be a down hill battel, or include defesive bounses for acrose rivers or any thing for that matter?
a simply way to implament flanking is like this..
if a unit is atacked from the point a one turn this will decied witch derection the defending units faceing. then in the same turn if a unit atacks from point b, its side. the unit will not be able to move the bulk of its forces thear to defend so the atacking unit will have a 25% atvantage. now if another unit in the same turn atacks from pont c *defending units rear* it will have even less forces ther, so it will be a 50% atacking atvantage. so the faceing of a unit is decited by whear its first atacked, on faceing the first atacking unit, its no longer able to turn in any direction to meet new threats.
if the first atacking unit atacks, then retreats or leave the space nest to it, ther shold be no flanking bounus. so for the bounus to work, the first atacking unit must remain in the squer whear it atacked the defending unit.
now if a unit is completly surounded, the unit shold suffer a 50 defensive penalty, no mater witch side its atacked from. and a 25% atacking penalty.