south end of the island. Keep The rectangular keep is on plan, and high. Inside, there were five storeys. The single entrance to the keep, on the east side, was accessed from a wooden stair-tower via a movable bridge. This entrance is at first-floor level, and led into a reception hall. Also on the first floor was the kitchen, and below this, accessed by a ladder, were vaulted storage cellars with a well and a prison. From the reception hall a spiral stair within the -thick wall led up to the great hall, and above this the lord's outer chamber and bedchamber. Larger windows are provided on the less vulnerable west and north sides of these rooms. The top level was used as servants' quarters or, during times of trouble, as accommodation for the garrison. The battlements were accessed from this floor, and nine windows gave views in all directions. An outer door at this level allowed equipment to be hauled up from ground level. On three sides, holes in the external walls would have supported a timber gallery known as a bretèche, which allowed the defenders to drop objects on attackers at the walls, while a more permanent machicolation over the entrance served the same purpose. Curtain wall and other features The keep is enclosed by the later curtain wall, which wraps around the east, south and west sides of the keep. The east and south sides form the "artillery house", begun by the 8th Earl of Douglas in 1447 and representing a sophisticated artillery defence for its time. It faces the higher ground to the east of the river, from where an attack was most likely to come. It has round towers on its three corners, which are provided with three gun ports on two levels, with a crenellated parapet on the third level. The gun ports on the lower level are 'dumb-bell' shaped, while those above are of the 'inverted keyhole' type. It has been conjectured that the guns mounted in these openings would have been a breech-loading device of around calibre, attached to a wooden stock. Only one of these towers, the south east, remains, while the others were damaged in the 1640 siege and subsequently collapsed. Between the towers is a curtain wall, thick and formerly up to high. It is battered to provide greater strength against artillery, and provided with traditional slits for firing