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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The Church of St Donatus name refers to Donatus of Zadar, who began construction on this church in the 9th century and ended it on the northeastern part of the Roman forum. It is the largest Pre-Romanesque building in Croatia.
The beginning of the building of the church was placed to the second half of the 8th century, and it is supposed to have been completed in the 9th century. The Zadar bishop and diplomat Donat (8th and 9th centuries) is credited with the building of the church. He led the representations of the Dalmatian cities to Constantinople and Charles the Great, which is why this church bears slight resemblance to Charlemagne's court chapels, especially the one in Aachen, and also to the Basilica of San Vitale in Ravenna. It belongs to the Pre-Romanesque architectural period.
The circular church, formerly domed, is 27 m high and is characterised by simplicity and technical primitivism.