Dudeldorf Castle is the most important monument in the parish of Dudeldorf. It was built in 1345 and restored 1451–53.
The original Dudeldorf Castle was built in the 12th century, although the exact date is not known. A lord of Dudeldorf is recorded in 1052. In the 14th century this aristocratic family became extinct.
The castle occupies the northern part of the village walls which also form the curtain walls of the castle. The overall site consists of the medieval tower house, which was converted into a school in the 19th century, the double-winged manor house, added in the 18th century, the courtyard with its archway and the garden and pavilion. The bergfried, which rises above a vaulted basement, bears the date 1734 at the level of the uppermost of its three floors.
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.