Portencross Castle lies on the seaward side of the settlement of Portencross (North Ayrshire), 2 miles (3 km) northwest of West Kilbride. The 14th century tower occupies the site of an old Roman fort, situated on a rocky promontory which extends out into the Firth of Clyde. It comprises an oblong structure of three storeys, built in red sandstone, with an attached four-storey wing to the east.
Originally belonging to the Ross family, they supported the Comyns against Robert I during the Wars of Independence and the victorious King gave the castle to the Boyds of Kilmarnock. Robert II most likely stopped here en route from Rothesay Castle to his coronation at Scone in 1371 and Robert III also stayed here evidenced by charters signed while resident. It remained the property of the Boyd family until 1785 when it passed to the Fullertons. It was used in the 18th century to house French prisoners of war, but lost its roof in a storm in 1739. Although now uninhabited and ruined, it still stands to its full height.
Portencross is currently owned by British Energy Plc, who operate Hunterston 'B' Nuclear Power Station which lies 1½ miles (2.5 km) to the north, although the intention is to transfer ownership of the castle to the community. Plans are in place to restore this A-listed scheduled monument, at a cost of at least £800,000, and open it to the public. The castle was featured in the BBC television series Restoration (2004).