King John's Castle, also called Carlingford Castle, stands on a rock above the harbour, commanding the narrow pass between the mountains and the lough. Its west wing was built by Hugh de Lacy, Lord of Meath, in the late 12th century and the east wing was added around 1261. It is a D-shaped enclosure castle with walls 3.4 metres thick, and it changed hands repeatedly in the wars of the 17th century; before the Battle of the Boyne the Williamite general Schomberg used it as a hospital.
The castle itself is closed to the public for safety reasons, but there is a viewing area on its eastern side looking over the north pier and the lough. Its silhouette against the Mournes is the classic Carlingford view.