Built during the reign of King John (1199-1216) by Richard de St Michael, who had been created Baron of Rheban, this fortress was one of two castles constructed to defend crucial river fords in the area; its twin, Woodstock Castle, guarded another crossing point nearby. The strategic importance of Rheban is underscored by a royal visit in 1288, when John de Sandford, Archbishop of Dublin and the king’s justiciar in Ireland, came to inspect the fortification.

What survives today is a complex architectural puzzle that spans several centuries. The original 13th-century structure consists of a long, rectangular masonry building measuring nearly 14 metres in length, with walls almost two metres thick. The ground floor contains two parallel barrel-vaults, each with defensive arrow loops that once allowed archers to protect the approaches. A narrow intramural staircase, now blocked, originally provided access to the upper floors. The building shows clear signs of later modification, including 17th-century additions that transformed the medieval castle into a fortified house, complete with a small rectangular tower attached to its western side.

The castle’s defensive features tell a story of evolving military architecture and changing needs over time. The original structure’s thick walls and chamfered base-batter were typical of 13th-century defensive design, whilst the later additions reflect the different requirements of 17th-century warfare and domestic comfort. Though heavily overgrown with ivy and partially ruined, the building still reveals fascinating details: barring-holes for securing doorways, wicker-centred vaulting, and various blocked passages that hint at the castle’s complex internal layout. Archaeological surveys in 1987 even noted traces of a defensive fosse, or ditch, about 40 metres to the west, though this feature is no longer visible at ground level.