Oillet

[Jason Yan]

A small opening or loophole, sometimes circular, used in
mediaeval fortifications.

[Dave Paprocki]

In the times of [castle] warfare, these were windows that went through thick walls of the front of [the castle] where the window was wider on the inside than on the outside. A benefit to such windows is it was easy to fire an [arrow] out, but archers on the outside would have trouble firing back at you.
A word that rhymes with toilet.

[Dianne]

An aperture or loophole for observation, especially in a wall; = eyelet. Now (Architecture): a round opening at the end of an arrow-slit (historical).