My proposition is: add a special brick or wooden plank with an arrowslit.
It would be an advantage to defending archers on attacking archers. Like in medieval castles actually.
But it may be too powerful. I let you decide
Did you know that in french, these are called "meurtrières"? That essentially translates to "murder holes".
edited January 2017
Actually, for the story, I only knew the french word and had some difficulties to find the accurate english word for it because google translate tells me non-sense, a meurtrière being just a murderess, I then managed to find "murder hole" which represents a kind of "meurtrières", but not the kind I was interesting in.
In the same theme, but for attackers, another thing that was greatly used in medieval sieges, is the
. It would be in wood only, so less resistant than a stone arrowslit, very slow to move but allow an archer to shoot without being easily killed.
I second this idea. Having a brick or half-bricks you can use to add arrowslits to that wall would be great. I don't think they would be to over powered, just have the arrowslit bricks be a bit weaker and easier to destroy than regular bricks. Also just like real castles the walls were thinner with the alcoves that the arrowslits were in, so that could balance out their utility.