Royals, like everyone else, call toilets by their common names: toilets, bathrooms, restrooms, lavatories, and so on. There's no special royal term for a toilet.
While some might imagine extravagant names or euphemisms, the reality is that even in palaces and castles, basic necessities like toilets are referred to with everyday language.
However, there might be some historical or cultural nuances in how royal households refer to toilets. For instance, in older times, toilets might have been referred to as "privies" or "water closets." But these terms are generally not used today, even in royal circles.
The use of specific terminology is often dictated by tradition, social norms, and personal preferences, not necessarily by royal status.
