An arrowslit is a slit, such as can be seen in a castle wall, through which arrows can be fired.
An arrowslit or arrow loophole is called "une meutrière" (fem.) or "une archère" (fem.) in French.
yes, infact it is shaped like a keyhole, or atleast very large one. the reason being so that the person firing the arows will not get hit with other arrows or another type of projectile.
Perhaps an arrowslit or "balistraria" Or maybe you should put your character on top of the castle walls so you can talk about the battlements or "crenellations".
The slits are simply called arrow slits or occasionally Balistraria. Sometimes the arrow slits are looking out of embrasures. Arrow slits were useful for protecting the castle as they were easy to fire out of but very, very hard to fire into. To find out more about embrasures click on the link below. "Wikipedia.org: Embrasure" To find out more about arrow slits click on the other link below. "Wikipedia.org: Arrow slits".