One side of the courtyard of our hotel is a part of the medieval walls that protected Jerusalem.  We can sit in a little nook and look out over the streets and shops.
The slit in the wall is called an arrow loop or slit, a thin opening with a bevel inside providing enough room for someone to shoot a weapon out the slit. Archers would be able to shoot at invading armies.

Today it protects us from the sun gives a narrow view of life outside the walls.