Los Arcos Castle was built around 1474. The square form castle has four corner towers and it was surrounded by a moat. It was built of masonry, dressed stone and brick, as a rectangular enclosure with squat towers at the corners and a keep in the centre. It is not open to visits at the present as it is private property and currently inhabited.
Near the castle they have found the remains of an old Gothic-Mudejar hermitage or basilica and the medieval Castillejo bridge.
Towering 52 meters above the sea, Bengtskär lighthouse is the tallest one in Scandinavia. The building started in in 1905 after the shipwreck of S/S Helsingfors and was completed in 1906. The lighthouse was designed by architect Florentin Granholm. On December a special petrol lantern, designed and built in Paris, was brought to Bengtskär and installed atop the tower.
German fleet bombarded Bengstkär in the First World War in 1914. Since the Gulf of Finland was heavily mined, it was not until 1919 that the surrounding seas were declared safe for shipping, that the light was lit again.
After the war the military value of Bengtskär increased as part of the defence system of independent Finland. In Second World War (1941) Soviet Union made a suprise attack to island. After a bloody battle, the small Finnish garrison emerged victorious. Intermittent repairs to the facility continued during the post-war period.