The fortified town of Ostrovica is located on the hill of the same name, on the left bank of Una, above Kulen Vakuf. The first records of Ostrovica date back to 1407, when Ladislaus, king of Hungary and Croatia, confirmed that the fortress was the property of duke Sandalj Hranić. The fortress came into Hranić's possession thanks to the dowry of his wife, Katarina Vukčić Hrvatinić. Seeing as Katarina hailed from the Hrvatinić family of Bosnia, we can assume that the fortress was built by a member of this aristocratic family some 500 years ago. It was probably built by Hrvoje Vukčić Hrvatinić, who also built Jajce. 

Due to the fact that it was an exceptionally important border fort, Ostrovica was continually expanded by towers, bastions and ramparts as much as 10 meters high. 

The importance of Ostrovica is also demonstrated by many historical monuments scattered in its immediate vicinity. The royal town of Ostrovica forms a part of Kulen Vakuf, a town lying on both sides of the river of Una. The town's strategic location and the fact that it is virtually inaccessible to the enemy on all sides further corroborates the theory that it was visited by Bosnian kings.