Dominating the Monte Boi promontory, this fortress was erected between the 11th and 17th centuries and its impregnable 3km circle of walls still stands, though there's not much old left inside it now. An enticing 40-minute walking trail loops round the rocky shoreline, broken up by a few small beaches.
Within the precinct today is the luxurious, medieval-castle-style Parador de Baiona. Have a drink on its cafe terrace, with fabulous views across the bay.