It has two metre thick walls in places and contains three phases of building. The original structure built in the 14th century consisted of the entire curtain (enclosing) wall and three polygonal towers. The square tower located along the north wall (that parallel to the road) was probably added in the 15th century, while more rebuilding and refurbishment took place in the 17th century. It was constructed of cut sandstone blocks and rubble. The main entrance was through a three-storey gatehouse at its western end which consisted of twin polygonal gate-towers which led to an internal courtyard. At first floor level to the east is the upper ward of the castle in which was probably located the great hall.

Although devastated by the wages of war and decay of time, this castle in terms of scale and construction compares with the great Edwardian Castles of Wales (it has been suggested that it was built by an expert who was involved in the erection of Harlech and Caernarvon Castles).

Captured by the Scots in1316, during the invasion of Ireland by Edward Bruce. On the defeat of Bruce it reverted to the Earl. The Red Earl's grandson, William, who succeeded the earldom in 1328, was at once in conflict with a distant cousin, Walter de Burgh from Connacht. He had Walter arrested and brought to Northburgh where he was slowly starved to death. The savagery of this act prompted the Mandeville family on the instigation of Walter's sister to murder William in 1333.

As there was no one other than a young daughter named Elizabeth to take up the Earldom (and as she fled to England along with her mother), Anglo - Norman power in Ulster was brought to a temporary end. The O'Doherty's (dependants of O'Donnell) took control of the castle.

However, fighting within the O'Donnell clan resulted in a conflict during which help was procured from the Scotch and the castle was badly damaged by cannon in the ensuing war.

The castle then fell into the hands of Chichester during the plantation but eventually fell into disuse. The castle itself should be viewed right up close and especially to the shore-side where the scale can be appreciated. The rock out-crop upon which part of the walls and towers were constructed offers an unrivalled view of Magilligan Strand on the other side of the Foyle.