Loop hole
A balistraria is a narrow opening, often of a cross shape, through which arrows can be fired.
Castle loop holes are narrow vertical slits in the castle wall that allowed bowmen to shoot arrows at the enemy while making it difficult for the enemy to shoot into the castle.
The slit windows or archers' windows of medieval castles were also called loopholes.
narrow
A narrow opening can be considered a slit. Narrow openings can be very long or short. There are many different types of narrow openings.
Slot
The pharynx is the narrow opening at the back of the mouth. It is part of both the digestive system and the respiratory system.
The things you call "windows" are actually arrow slits. They are just wide enough to fire a arrow through, but if you went inside the area where the windows are located in the wall you would find a recess for a man to use to aim, spot, and fire an arrow. The slits look fairly narrow from the outside, but they have a full view of the land below from all angles. This is very good for the defense of the castle.
spinal cord
The shape of the opening. A Beaker has a wide opening, the curved sides of the beaker do not converge. A flask has a narrow opening, the curved sides form a narrow neck, that can be bunged.
Chanelled into a narrow opening.
crack!