The original castle replaced the fort in 1604 and was erected by Lord Mountjoy. The castle was built as much to defend Mountjoy against the citizens of Cork (who had showed rebellious inclinations the year before when they refused to acknowledge King James I as their new monarch following the unlamented death of Queen Elizabeth) as against the threat of a Spanish invasion. The fear of the Spaniards was still very much alive following the Battle of Kinsale and provided the incentive for the construction of this and similar forts to defend the water approaches to the city.

The castle, now a simple circular tower, was returned to the City by James I no later than1608. The structure was used as a light beacon to guide shipping and was fuelled by burning truf,. In 1722, the tower was destroyed by fire, but was rebuilt by the citizens of Cork. The new castle included a circular tower described as a 'handsome octagon room' crowned by a cupola and flagstaff and again included a light beacon on top of an adjoining cylindrical tower. This second building was again destroyed by fire in 1827 and was rebuilt in 1829 by the city.