It is a rectangular Tower house four storeys high, i.e. four superimposed single chambers each about. On the north face are two rectangular projections, one containing a stone spiral staircase, the other an inner closet at each level, with those on the lower stages having outlets to the ground. Architecturally there is little evidence to give a definitive date for the castle. The masonry is blue stone rubble with a little freestone in quoins and window jambs. Some of the window details suggest 15th century, but have had so much reconstruction that dating is difficult. The ground floor chamber is unfloored and the irregular surface of the outcropping rock can be seen. It therefore may have been a storehouse. The main room is apparently on the first floor, which contains its original stone floor supported on a pointed barrel vault. The floors on the second and third storeys are at the original levels but of modern construction, with the beams going at right angles to the original direction. The concrete roof is also an addition. The wall tops retain their stone-flagged rampart walls and archery turrets. The turret to the north-west contains a dovecote, the nest-holes of which are contemporary with the main structure. The entrance is at the bottom of the north-west tower and leads to a spiral stairway to roof level. It is protected by a machicolation at that level. The projecting towers are connected by a high-level arch which also functions as a machicolation. The Dublin Penny Journal of 30 March 1833 describes Jordan's Castle as follows:
History
Its early history is somewhat obscure. The earliest authentic reference is to a defence of the castle by Simon Jordan against the O'Neills for three years, until relieved by Lord Deputy Mountjoy in 1601. The 18th century historian, Harris, says that "it is most probable that Jordan's Castle was erected by one of the family, whose arms are fixed in a stone near the top". In 1911 the Belfast antiquarian, Francis Joseph Bigger, bought the castle and restored it, using it to display his extensive collection of antiquities and making it freely accessible to everyone to bring local people 'into touch with the Irish past, and give them some conception of the historic background of their life'. It was also extensively used for get-togethers of the wide circle of Irish Cultural revival artists and writers to whom Bigger was friend and patron. Following suggestions by Alice Stopford Green and the archivist Henry Egan Kenny, Bigger renamed the tower "Castle Seán' in honour of the two years when Shane O'Neill controlled Ardglass, and they believed, re-fortified the tower following his defeat of the MacDonnells at Glentaisie in 1565. When he died in 1926, the castle was presented by his executor, Dr Joseph Bigger, to the state on condition that, with its contents, it should be preserved as an Ancient Monument. The contents have since been dispersed among the Ulster Museums general collections and the tower is no longer open to the public. Ardglass had at least six castles and remains of four of them can still be seen: Ardglass Castle, Cowd Castle, Margaret's Castle and Jordan's Castle.
