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.

Historically, the official keeper or resident governor of a castle is called a Castellan. This role was vital in the Middle Ages, as the Castellan acted as the lord's representative, managing the castle's defense, maintaining the structure, and overseeing the local "ward" or territory. In some regions, the title might be Constable or Governor. In 2026, many historic castles still maintain honorary or functional keepers. For instance, the Tower of London has a "Resident Governor," while other British heritage sites employ a Headwarden or "Custodian." These modern keepers focus on historic preservation and tourism management rather than military defense, though they often still live within the castle walls to ensure 24/7 security of the priceless heritage.