Prominently sited on a rocky outcrop close to Greencastle Point, the castle commands the narrow entry to Carlingford Lough and is within sight of Carlingford Castle. A royal castle, built in about 1250, it had an eventful military history. Attacked and damaged by Brian O’Neill and Hugh O’Connor in 1260, besieged and taken by Edward Bruce in 1316, attacked and spoiled by the Irish at least twice in the later 14th century, it was still maintained as a garrison for Elizabeth I by Bagnall in the 1590s. Coastal patrols, contact with Dublin and the trade of produce from the area made the two castles an important strategic unit.
The castle is approached across an impressive rock-cut ditch, partly excavated and left open. The curtain wall with four corner towers enclosed a trapezoidal area but is badly ruined. The best-preserved stretch is visible in the farmyard on the south-west. The north-east tower was excavated and conserved and part of the east curtain, found collapsed intact into the ditch, was reconstructed near the car park. The narrow wall crossing the ditch was intended as a dam, but the rock would not hold the water. The large rectangular keep or hall is of the 13th century but with substantial 15th- and 16th-century alterations. It was originally entered by a first floor door on the south, protected by a forebuilding (excavated foundations visible). A ground floor door in the west wall is a 15th-century alteration and the rough gap near the south-east angle (the present entrance) is a late forced entry. The cross walls dividing the ground floor into three vaults are also 15th-century insertions. At first floor level was the great hall (late medieval windows and fireplace) with a latrine in the north east corner. The upper parts of the hall, with mural passages, wall-walks and angle turrets, date from a 15th-century remodelling.
Fragments of other buildings in the ward include part of a long rectangular structure south-west of the keep. A long series of excavations has been carried out here as part of the conservation programme. A motte and a church ruin survive nearby.
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