The author used the expression "wall-eyed" to describe a lunette, an arching aperture in a wall that may function as a window. Wall-eyed means something or someone has bulging eyes, or eyes that show a great amount of white for some reason. With that in mind, we can think of a couple of things that may be described as wall-eyed. A fish, for instance, can be wall-eyed due to its bulging eyes that seem to be constantly staring. A mirror reflecting a white wall, a round window reflecting moonlight, all can be described as wall-eyed. The same goes for a person who is surprised or scared, since they will open their eyes wide.
