http://en.wikipedia.org/wiki/Arrowslit Not entirely sure what impact windows and doors (let alone low walls) have on archery exactly; however, even an arrow slit can be fired into from the outside. Better defense to peeps inside than the aforementioned walls and fences... blah blah. Offhand would say stone only; though, the other wall material types would probably be fine. EDIT: Oddly enough, dont see this come up often.