The civlopedia, among its other lies, says that units melee attacking from a hill will be more effective against units in open terrain. Perhaps the devastating two thirds strength death sentence we see applied to all units in open terrain was only intended to apply in this special case of units charging down a hill, but is applying to all units in open terrain because Firaxis has crappy QA. I think this would be balanced, flavorful, fun and add a lot of tactical depth. As it stands defenders have a disadvantage in war, which is just stupid. The civlopedia also implies that cities can be subjected to flanking bonuses, which is false. I think that would be neat, but cities already need a strength boost - if this was implemented they would need it that much more desperately. I'm kind of disappointed that I have to choose between conquering the world or playing a challenging game.