Fortress Kostajnica was built most probably in the 14th century and owned by the members of the noble families Frankopan, Lipovečki, Tot, Benvenjud and finally (in the 16th century) Zrinski, so it is today also known as Zrinski fortress or castle. It was conquered by the Ottomans on 17 July 1556, but it was freed from their rule relatively fast, in 1688.
Situated on the bank of the river Una, the fortress has very strong walls and three towers. Having been in pretty bad shape before, it has been renewed in the recent years by the Croatian Ministry of culture.
References:The Château de Foix dominates the town of Foix. An important tourist site, it is known as a centre of the Cathars. Built on an older 7th-century fortification, the castle is known from 987. In 1002, it was mentioned in the will of Roger I, Count of Carcassonne, who bequeathed the fortress to his youngest child, Bernard. In effect, the family ruling over the region were installed here which allowed them to control access to the upper Ariège valley and to keep surveillance from this strategic point over the lower land, protected behind impregnable walls.
In 1034, the castle became capital of the County of Foix and played a decisive role in medieval military history. During the two following centuries, the castle was home to Counts with shining personalities who became the soul of the Occitan resistance during the crusade against the Albigensians.