Kreuzberg was first mentioned as Cruceberg in a document in 893. Around 1100, the settlement came under the ownership of the Counts of Are, who built an initial residential house for one of their administrators on Kreuzberg. It is often stated that the bergfried (keep) of the castle was built during the 15th century. However, it is quite possible that this part of Burg Kreuzberg belongs to the oldest section of the complex and dates back to the 14th century. French troops destroyed the castle in 1686.
Today the main castle rises on an almost triangular ground plan, following the structure of the artificial castle plateau. The area is surrounded by the inner curtain wall. On the north side of the main castle area stands the baroque, two-story residential building made of rubble stones.
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.