The castle was built in the 13th century, possibly by William, Earl Marshall. Originally, the castle formed a square, with large corner towers. Only half of the castle now remains. The most complete tower contains a fine circular chapel, with carved ornament. The tower also has several original fireplaces and a vaulted basement. Archaeological excavations revealed a rock- cut ditch outside the castle walls.
Ferns Castle is one of a number of historic sites in Ferns. Others include St. Mary’s, a 12th century Augustinian Priory; the remains of a 13th century cathedral, part of which is incorporated into the present Church of Ireland Church; St. Peter’s, a small nave and chancel church; and some High Crosses and parts of crosses, which stand in the cathedral grounds.