Well, a hole in a wall, goes all the way down to the floor so not a window. A bit wider than the usual 80 cm minimum width for a doorway, tho. But not by much.
For a long time, I thought they blocked off this hole-in-the-wall somehow to create Garrovick's quarters in "Obsession". They didn't; they just hid things in violet shadows. The doorway has never been closed by anything AFAIK.