Dunoon Castle is a ruined castle located at Dunoon on the Cowal peninsula in Argyll and Bute, Scotland. The castle sat upon a cone-shaped hill of about 80 feet high, a volcanic plug. Very little remains of the castle's structure today. Castle House, built in 1822, stands a few yards north of the castle ruins. The remains of the castle, and a surrounding area, are a scheduled monument. The castle was a royal residence in the 14th century, and in the 17th century fell into ruins.