Bjelaj castle, locally known as Stari Grad Bjelaj (Bjelaj old town), is a medieval town-fortress complex near the village of Bjelaj, Bosanski Petrovac.
Located on the edge of the Petrovac plain (or part thereof Bjelajsko fields), on the northern slope of the Osječenica mountain. The area around the city is uninhabited and lean. The whole complex is situated on a plateau about 850 metres long.
On the south end of the northern half, there is a great fort, a prehistoric site from the Bronze and Iron Age. On the north end, there is a small fort. The fort itself has two parts: a medieval southern and northern Ottoman part. The medieval town has a ground plan of an irregular rectangle over 40 meters long and a width of about 35 meters. From the tower on the west side is the beginning of the remains of a large pen that was added along with the medieval city in the Ottoman period.
First mentioned in written sources in 1495 and named after the whiteness (or tint) by which it stood out above Bjelajsko field. Between 1530 and 1537, Bjelaj came under Ottoman rule.
Bjelaj has two parts: the southern medieval and northern Ottoman part. The medieval city has a layout of irregular rectangles over 40 m long (north-south) and about 35 m wide (east-west). The entrance to the city is in the northward.
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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.