To me a window does not necessarily have to be a glass object placed on a wall. A window can be an entrance, an opening, a gap, or a hole. I think a window is something you look through to see a different area. One example is a sewer. A sewer is an opening from the street to the under ground sewage. You can look through it to see a totally different area.
You stare intensely into them someone’s eyes. You are looking directly into someone’s soul. That is an example of a mental window. Your eyes are the window and their soul is what you are looking at. There for, to me there really are two different kinds of windows.