King John's Castle, Medieval castle in Carlingford, Ireland

King John's Castle is a D-shaped enclosure castle on Carlingford's harbour, halfway along the south coast of Carlingford Lough. Its curtain walls are 3.4 m thick, with a gatehouse and a square flanking tower in the west wing, and deep embrasures holding narrow arrowslits. The east wing, added around 1261, contains a large rectangular hall with two main floors over a basement.

Hugh de Lacy, Lord of Meath, built the west wing in the late 12th century, shortly after the Norman invasion of Ireland; other accounts credit Ralph Pepper in 1204. King John is said to have stayed three days in 1210 while campaigning against Hugh de Lacy, 1st Earl of Ulster. The east wing was added around 1261. The castle was held by the Irish Catholic Confederation and taken in turn by Sir Henry Tichborne in 1642, Murrough O'Brien in 1649 and Charles Coote in 1650, then fired on by Jacobites in 1689 and used as a Williamite hospital before the Battle of the Boyne.

The name comes from King John, Lord of Ireland, who supposedly stayed three days in 1210 while campaigning against Hugh de Lacy; local lore claims he began drafting Magna Carta here. The Irish name is Caisleán Rí Eoin. The castle is a National Monument and is closed to visitors.

The castle is closed to visitors, so it cannot be entered; conservation work was undertaken in the 1950s. It stands at Carlingford harbour, halfway along the south shore of Carlingford Lough, and can be viewed from the outside. Plan around the closure rather than around weather or footwear.

Local lore holds that King John began drafting Magna Carta at Carlingford during his three-day stay in 1210, though the charter was sealed in 1215. Other accounts credit the castle's construction to Ralph Pepper in 1204. The castle has been closed to visitors since conservation work carried out in the 1950s.