Medieval castles flew flags from their highest points for one of two reasons: either the flag was representative of the family who owned it (most often royal standards of heraldry), or to display the country to which they belonged.

In the medieval era, flags flying atop castles served three critical purposes: identification, communication, and psychological warfare. First, the flag (or Royal Standard) displayed the heraldry of the lord or noble family currently in residence, acting as a visual "ID card" for approaching allies or messengers. If the lord was away, the flag might be lowered or changed. Second, flags were used for long-distance signaling; different colors or positions could warn neighboring towers of an approaching enemy or request reinforcements. Finally, flying a large, vibrant flag was a display of prestige and defiance. It signaled to potential invaders that the castle was occupied, well-resourced, and ready for a siege. After a successful conquest, the first act of the victor was almost always to cast down the old flag and raise their own, a symbolic "refresh" of the landscape that signaled a change in power to the entire surrounding countryside.