Greencastle is a castle in County Down, Northern Ireland. It is a royal castle built in the 13th century. It was attacked and taken by Edward Bruce in 1316, attacked at least twice by the Irish in the later 14th century but still maintained as an English garrison in the 1590s. The large rectangular keep with three vaulted chambers at the ground floor is approached across a rock-cut ditch. While it dates mainly from the 13th century, it had substantial 15th- and 16th-century alterations.