The old town of Kostajnica became a fortified town in the 15th century, when it was owned by the Arlandović family. Martin Frankopan obtained possession of it in 1442. In the first half of the 16th century, with a grant from the Hungarian king Ferdinand, it came under the rule of the Zrinski family, and already in 1556 it fell into the hands of the Turks, who served as a stronghold for the defence of the Una border until 1689. The town was built along the river in the form polygonal building on stone foundations. It has a strong semi-circular tower with loopholes and two quadrangular towers.
The project of complete renovation and presentation of the Old Town in Kostajnica started with structural renovation, grouting and partial rebuilding of the walls of the pentagonal tower. The walls were plastered and whitewashed with lime according to the preserved original parts. The renovation of the roof structure was carried out according to the original geometry and by replacing the cover made of hand-hewn larch shingle boards. The facade remained stone. Repair work was carried out on the outer walls and the decoration of the Old Town from the inside with a promenade along the ramparts.
