What do we call a castle’s defensive wall openings between merlons?

The story behind the answer

In a castle parapet, the alternating high merlons and low gaps form the crenellation; the gaps are the crenels (crenelles). They let defenders shoot and observe, then duck behind the merlons for cover. By contrast, battlements refer to the entire parapet, embrasures are splayed wall openings, and loopholes (arrow slits) are narrow vertical cuts through the stone.

The term comes from Old French crenel, and “crenellation” was so tied to fortification that medieval English kings required a licence to crenellate before a lord could add them. Regional styles varied—Italian cities famously used swallowtail merlons—while features like machicolations projected outward to drop stones or boiling substances.