Walls: Bane to all Xbowmen and Gunners

It's been really bugging me that when I put gunpowder units or xbowmen on a wall they tend to be close to useless and this of course was seriously not the case in actual medieval times hehe. Sooo I was curious if there was some known way to make the top of the walls not impede your ranged unit fire BUT impede enemy fire, while this would not be entirely realistic it would fix my lil problem and make me tremble at the thought of approaching a wall lined with crossbowmen, usually I just laugh at the computer and slaughter the poor lads far too easily :(

(If there is any mod that already does this I would be grateful if someone pointed me to it, but I did check extensively and havent seen one so I might have just missed it)