The castle was probably last used militarily during the time of the French occupation of Jersey (1461–1468, when the French held the east of the island, but loyal Jersey folk held the western parishes). In 1483, after several years of petitioning King Edward IV, the Seigneur of St Ouen - recommended to the King's Grace by his father-in-law the governor, was granted a ″License to Crenelate″ - that is allowed to fortify his manor house. Governor Harliston approved the dismantling of the ″fort of refuge″ at Grosnez so that the already-worked stone could be re-used in fortifying St. Ouen's Manor.
The castle was likely last employed militarily during the period of French occupation of Jersey (1461–1468, when the French controlled the eastern part of the island, but loyal Jersey residents maintained control over the western parishes). In 1483, after several years of pleading with King Edward IV, the Seigneur of St Ouen - endorsed by his father-in-law the governor, was granted a ″License to Fortify″ - that is allowed to strengthen his manor house. Governor Harliston approved the dismantling of the ″stronghold of refuge″ at Grosnez so that the already-harvested stone could be reused in fortifying St. Ouen's Manor.