I do think the person above you is correct in that the author of the blog made a mistake. I've never seen arrow slits that are narrow inside the wall.

If I look at the old wall around my city's medieval centre we can clearly see they this:

From the outside: https://www.google.com/maps/@50.845463,5.6946097,3a,75y,330....

Inside: https://www.google.nl/maps/@50.8456076,5.6945364,3a,75y,165....