Château de Montguyon, a powerful castle since the 11th century, was first mentioned in 1080. In 1404, Rosine de Montaut-Mussidan brought it as dowry to Guy II de La Rochefoucauld, and the La Rochefoucauld family owned it until its sale in 1683. The fortress played a role in history, such as being liberated by Dunois after the Battle of Montguyon in 1451. It served as a Protestant stronghold during the Wars of Religion and hosted notable figures like Henri de Navarre (future Henri IV) between 1571 and 1586. In 1621, King Louis XIII and Anne d'Autriche stayed at the castle.
In 1683, Pélagie de Rohan-Chabot acquired the barony, and it remained in the Rohan family's possession until the French Revolution. Struck by lightning in 1793, the castle became a ruin sold as national property.
Built on a rocky spur with remnants of a drawbridge, Montguyon Castle consists of a main building and a tower from the 13th century, modified in the 14th century. Originally reaching a height of 50 meters, the tower now remains in ruins after the 1793 fire. A section of the keep collapsed in 1982 due to an earthquake. Inscriptions of maxims have been discovered on the walls of the second floor.
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.