Allentsteig Castle, located on a rock to the north of Allentsteig and bordered by a former moat to the east, was founded by the Kuenringers around 1000. It remained in the possession of the Lords of Kamegg-Kaya until about 1390.
Around 1500, the Hager family received the estate and transformed the medieval castle into a Renaissance palace between 1544 and 1570. Sigmund Hager, a prominent Protestant, visited Luther in Wittenberg. Paris von Sonderndorf acquired Allentsteig in 1599 and, due to his opposition to the Emperor, the town was plundered and occupied during the Thirty Years' War. In 1629, Hans Friedrich von Sonderndorf lost his estate, which was then acquired by the Rappach family. By marriage, it passed to Count Ernst August von Falkenhayn.
Fires in 1682 and 1752 caused significant damage. In 1804, Baron Leopold von Hahn bought the estate, and it later passed to the Pereira-Arnstein family and the Princes of Liechtenstein. Baroness Maria von Preuschen inherited it in 1918. When the German Reich established the Allentsteig military training area in 1938, the castle became the command center, a role it retained under Austrian control after the Russian occupation from 1945 to 1955.
The outer walls of the five-sided Renaissance structure largely originate from the medieval castle. The oldest parts include the massive keep from the second half of the 12th century with a Romanesque portal and the adjacent eastern quoin wall. The turreted top floor with a tent roof was added in 1904. The castle is accessed from the east through a two-story gate hall from the 16th century, featuring a wide bay window over a Renaissance portal. The inner gate wall, dating to around 1300, is pierced by a Gothic pointed arch from the early 14th century. The windows with profiled stone frames on the outer front date to the late 16th/early 17th century. The three-story rectangular arcade courtyard, marked with 1576, features flat arches on Tuscan columns on three sides. The interior was mostly remodeled in the late 19th century, including historicist tile stoves.
Currently (as of 2016), the castle houses the command of the Allentsteig military training area and the 4th Reconnaissance and Artillery Battalion.
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.