A castellan, or constable, was the governor of a castle in medieval Europe. Its surrounding territory was referred to as the castellany. The word stems from castellanus. A castellan was almost always male, but could occasionally be female, as when, in 1194, Beatrice of Bourbourg inherited her father's castellany of Bourbourg upon the death of her brother, Roger. Similarly, Agnes became the castellan of Harlech Castle upon the death of her husband John de Bonvillars in 1287. The title of "governor" is retained in the English prison system, as a remnant of the medieval idea of the castellan as head of the local prison.

Bourbourg is a commune in the Nord department in northern France. It is situated in the maritime plain of northern France, in the middle of a triangle formed by Dunkirk, Calais, and Saint-Omer.