Greencastle Royal Castle, located at Greencastle near Kilkeel in County Down, stands as a sentinel commanding the mouth of Carlingford Lough in Northern Ireland. Built in the 13th century, likely by Hugh de Lacy, the castle formed part of a defensive pair with Carlingford Castle across the water, guarding the narrow entrance channel and ferry crossing. The castle's most substantial surviving feature is a four-storey keep measuring 18 meters by 8.5 meters internally, originally surrounded by outer rectangular walls with corner towers. The fortress endured sieges in the 14th century and remained garrisoned until 1590, serving as an important military stronghold throughout the medieval and early modern periods. Though visitor access is limited, the grounds remain open regularly, offering views of this strategically positioned 13th-century fortress and its role in medieval Ulster's defensive landscape.
