The first castle in Xaintrailles is mentioned in 1259 as being held by three co-lords, all from the Xaintrailles family. It was built in the early 13th century, and what remains are the medieval outer courtyard, the keep, and the embankments of the south and west walls.
Jean Poton de Xaintrailles, a companion of Joan of Arc who died in 1461, followed by his successors Bernard de Lamothe and Antoine de Chamborel, governors of Casteljaloux, undertook constructions in the 15th century: two towers and the north residential wing. In the 16th century, turrets with fan-shaped projections were added to the south elevation, and moats and a drawbridge are mentioned in 1539. In 1570, the castle passed to the Montesquiou family, and later through marriage to the Lau de Lusignan family in 1724.
Around 1780, Armand de Lau, Marquis de Lusignan, had the moats filled in and an access arranged from the west. In 1824, the northeast rooms collapsed, and in 1843, the keep was partially restored based on the project of Gustave Bourières, the department's architect.
The oldest part of the castle is the 13th-century keep, with windows added in the 15th century. The north residential wing from the 15th century has one square floor and two attic floors with a hipped roof covered with flat tiles. Like the keep, it has an out-of-service staircase. The one-story and attic south building with turrets was renovated (kitchens). The west wing is flanked by two towers.
The U-shaped staircase and vestibule were added in the 18th century.
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.