Ernstbrunn castle became the seat of the Ernstbrunn branch of the Counts of Sinzendorf in 1592. After the last Sinzendorf died in 1822, it was inherited by the Köstritz branch of the ruling princely House of Reuss of whom it remains their main family seat until today.
The castle is an elongated structure with four ascending courtyards on the northern slope of the Semmelberg above Dörfles. The medieval core castle on the northern rocky spur was continuously expanded during the Late Renaissance, Baroque, and Classicism periods with new groups of buildings. The core castle, dating from the 12th and 13th centuries and surrounding the innermost courtyard, sits on a free-standing limestone rock on three sides, which drops steeply towards the village of Dörfles. It forms an elongated octagon. The wall thickness of the outer wall is up to three meters. It is further reinforced on the southeast by the square keep. Originally, this keep had only three floors but was extended by an additional floor in the 16th/17th century.
In front of the gate is an early Baroque fountain. This fountain has a total of 16 coat of arms representations, 4 at the top of the shaft below the fountain bowl and 12 in pairs around the edge of the hexagonal basin.
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.