Ruttenstein Castle was first time documented in 1209, in a deed issued by Duke Leopold for the Baumgartenberg Monastery, where it is mentioned under the name Rotenstein. The existing structure of the high castle, however, suggests that the castle is significantly older. The construction period likely falls in the first half of the 12th century, although this is not documented. It is probably a foundation of the Counts of Clam-Velburg.
The walls of the formerly massive pentagonal keep are now only partially preserved, tapering to a point and visibly towering over the site. The former residential wing in the west retains three of its walls. Above them, a viewing platform has been constructed, accessible via wooden stairs. Clearly recognizable, though also only partially preserved, is the early Gothic castle chapel located near the stone steps to the residential building. The main castle itself was protected on the north, south, and east by a mighty ring wall, which is still partially preserved today, and from which seven wall towers protruded.
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.