Mustio manor ("Mustio Castle", "Svartå Slott") was built in 1783-1792 by Magnus Linder, the owner of the local ironworks. There had been an older manor from the 17th century, but it was dismantled when the present one was built. The manor represents the neoclassical ("kustavilainen") architecture.
Today Mustio is a countryside hotel. There are also the old ironworks and one of the biggest private historical parks of Finland.
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.