Ok, this is a decent idea except for one thing. Most of us defend cities during this age with at least two archer/longbowman/crossbowmen units. If you're at war, you're going to put a few more defenders in border cities.
So, if the attacking Axemen get a +15% attack bonus, shouldn't the defending unit get a +15% defense bonus? In fact, if it is a city shouldn't every archer add his +15 defense bonus to the defending unit?
I guess my point is that if you're going to have a bonus for the attackers you need to add that bonus to the defenders.
Looking at it from a realistic point of view, most archer units would cancel each other out. Yes, the English Longbowman was an elite example, however they're not in the game.