Because the building is very old and the window doesn't seem to be a main one... wouldn't the window simply be for ventilation and a bit of light built in a time before larger panes of glass came in common use? It would make sense to have very narrow windows (the load problems above them would be reduced, for one thing) that would be the width of a pane of glass (4-6 inches). Also, I would think that a tall, vertical opening, if there was no glass, would take in less rain and snow.
If there was no glass used when it was built, what would have been the alternative to let in light and keep out weather? Slats, oiled hide?
I've always felt that the window tax had a kind of simple logic about it. The richer you were, the larger your house, thus the more windows. So it taxed the rich but not the poor.
In order to avoid it, the rich had to sacrifice some of their comfort, by bricking up windows.
OK - it's a highly moot point, but not entirely silly.