The ruins were acquired by the Earls of Bute in 1760, when John Stuart, the 3rd Earl and, from 1794, the 1st Marquess, married Lady Charlotte Windsor, adding her estates in South Wales to his inheritance. John's grandson, John Crichton-Stuart, developed the Cardiff Docks in the first half of the 19th century; although the docks were not especially profitable, they opened opportunities for the expansion of the coal industry in the South Wales valleys, making the Bute family extremely wealthy. The 2nd Marquess carried out exploration for iron ore at Castell Coch in 1827 and considered establishing an ironworks there.