The 12th-century late Romanesque church of San Juan Bautista is attached to a 10th-century Berber tower, which is 18 metres tall with a 9.4x7.4 metre floor plan and 1.75-metre thick walls on all of its three floors. Originally it was built with the rammed earth technique but then the structure was changed to stonework. The access door to the tower is hidden in a passageway that connects the tower to the church. Later on, the windows were opened up to place bells inside, and the tower was covered with a roof.
References: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.