noun
loophole; crenel; gun port; porthole
A narrow opening in a fortification wall for shooting through; a technical term used in military history and castle architecture.
城壁には銃眼がいくつも設けられていた。
The castle wall had several loopholes for firearms.
この銃眼から敵を狙うことができる。
You can aim at the enemy through this gun port.