After the fall of the Bosnian Kingdom in 1463 and the heavy defeat of the Croatian nobility at the Field of Krbava in 1493, the conquering Ottoman armies started to threat immediately the safety and existence of the lands of the Croatian Kingdom. The medieval development of Croatia was violently interrupted in all fields. The Croatian struggle for historical survival of the country and the nation lasted for more than two centuries, during the 16th and 17th century. In this struggle, the individual interests of the nobility and the common state policy of the Croatian Kingdom led by the Croatian Parliament permanently clashed The old feudal fortresses of the Croatian nobility had the most important role in the anti-Ottoman defense, especially in the southern highlands. The fortified feudal towns (castrums and citadels), the medieval centers of the feudal administration and judiciary, were turned into the fortified bastions to stop the Ottoman penetrations, plundering and destruction of the land. That was also the destiny of the feudal fortress Kostajnica on Una. The ownership of Kostajnica, the strong stone fortress built on a small island on the river of Una in front of the settlement bearing the same name on the left coast on the foothills of Djed Hill, changed many hands in the medieval period. In the 16th century, in the period of frequent Turkish raids across the river of Una, Kostajnica was feudal property of a very powerful feudal family of Zrinski, who also owned many other important fortified towns and settlements: Novi, Krupa and Liπnica on the river of Una, and Zrin, Prevrπac, Gvozdanski, Komogovina and Gorička in the area between Una and Zrinska gora. Kostajnica was the most important fortress on the rivers of Una between the royal fortified town of Bihać and the confluence of the rivers Una and Sava near Jasenovac. This is the reason why its owners, princes Zrinski did everything they could have to reinforce the fortress and to make it capable to resist the Turkish attacks. They kept their military crew in Kostajnica fortress as long as they could pay for it from the feudal revenues from their land and from the assistance they received from the king and other anti-Turkish European allies. Of course, this assistance was never sufficient, so Zrinski family tried to save their property in the Una basin by signing a separate peace agreement with the Turks. However, when they turned the policy of good neighborhood into the policy of active fight against the Turkish conquest, incited by the royal promise of the needed assistance, the Turkish armies almost completely destroyed their property in the Una basin in frequent attacks in the 1630s. In such a situation the Zrinski family was forced to cede their fortresses on the river of Una, including Kostajnica, to the defense of the royal army. The fortified bastion of Kostajnica was really the most crucial point of Croatian defense until it fell under Turkish rule. I happened in mid-July 1556, when the commander of the royal military unit, Captain Ignacije Lusthaller, shamelessly surrendered the fortress to the Turks; in accordance with the testimony of his contemporaries, for the prize of 2000 gold coins. After conquering Kostajnica, the Turks took over all other fortresses on the river of Una, and they made Kostajnica their strongest fortress from which all the Turkish conquering armies would set off towards the rivers Sava and Kupa, all the way to Turopolje and Zagreb until the end of the 16th century. Kostajnica as a Turkish stronghold had an extremely important role in the extensive Turkish offensive in 1577 and 1578. Until the liberation war in the end of the 17th century, Kostajnica and its fortress on the island remained under Turkish rule. The Turks (the Ottomans) in the meantime reinforced the old fortress with new annexes and kept in there permanently a strong military garrison. A few attempts of the Croatian Banate and Border-region armies to liberate Kostajnica, ended up in failure. Kostajnica was finally liberated by the joint forces of the Croatian Banate army and a part of the Austrian army that was led to Kostajnica by famous army leader Ludovik Badanski. It happened after a three-day siege on July 15, 1687. The Turkish military garrison was chased out of the fortress, and the garrison of the German Border Army was stationed there. After the liberation and the end of the Austro-Ottoman war, in accordance with the Peace Treaty signed Srijemski Karlovci on January 26, 1699, Kostajnica and its fortress on the island on Una became the part of Croatian Kingdom. However, that was also the beginning of the political fight between the Croatian Parliament and the Ban for the military control over this strategically important defense position at the Una borderline. Since there was an unit of German soldiers stationed in Kostajnica and paid by the Court War Council in Graz, the nobility of Styria tried to keep it under its control. On the other hand, the Croatian nobility tried to keep under its control the whole liberated area between the rivers of Kupa and Una, so that Kostajnica had a key role in this clash of political interests. The Vlachs settled in the area between Kostajnica and Zrin interfered in this clash of the military interests of the War Council in Graz and the political interests of the Croatian Parliament, and they took the side of the German military authorities. This clash was in the end solved in favor the Croatian Parliament and nobility by the decision of Emperor Leopold, stated in a document signed on May 21, 1695 in accordance with which Kostajnica is placed under the authorities of Croatian Parliament. Ban Adam Batthyani took over Kostajnica on behalf of the Croatian Kingdom. The German army leaves the fortress and the Banate army is deployed, under the command of the Captain of Kostajnica on behalf of the Ban. This was the crucial event in the process of development of the third border military region towards the Ottoman Empire, the creation of the Banate Military Border Region, which remained under the immediate authority of the Croatian Ban. Kostajnica, after being liberated from the Turkish occupation and after gaining a new political position, having such an important geographical position, develops in the course of the 18th century into significant communications and trade center. In one of the reports dating from that period of time it is described as follows: “Kostajnica is a small town on the island on the river of Una, linked to the river coasts with two bridges, and here the river is very wide, almost as wide as the Danube, although in other places Una is not wider than any of the small backwaters. The island is surrounded by the palisades, in the center of which there is a market town. In its upper part there is an old stone fortress, so called citadel with three strong, very narrow towers; in one of them there is a commander’s apartment made of wood, and currently inhabited by the Count Petar KegleviÊ, and in the other tower there is a military garrison. In the lower part (of the island) there is one very strong stone tower.” However, the civilian settlement was also developing under the hill on the left coast of Una, as written down by L.F. Marsili. “This town, almost the capital of the lower Croatian lands, is situated partly on the island on the Una, and partly on the coast, on the foothill. This is a new, now Croatian “townlet” of Kostajnica, as this settlement described by Marsili in his topographic map. On the top of the hill he marked the place on which a watchtower, i.e. smaller fortress should be built, in which the army would keep watch and guarantee a higher level of security to the settlement and to the fortress. Marsili’s drawing stresses the position of the Franciscan church and the monastery. As the settlement of the inhabitants and merchants of Kostajnica under the hill developed, the old fortress on the island was being reconstructed as of 1830s. In the mid-century a whole series of new facilities, so called “kostumaca” was built, and it would be used for the purposes of customs control of trade and the sanitary border regulations. This means that in the second half of the 18th century Kostajnica did not only have a military importance, but it also becomes a important communications ad trade center. Kostajnica retains such a historical position and the trend of development after the last Austro-Turkish war and final defining of the borderline between Croatia and Bosnia, in accordance with the Peace Treaty signed in Sviπtovo in 1791. Today the old fortress of the Croatian nobility on the island, the Turkish border citadel and the citadel of the Croatian Banate army are today historical monuments, witnesses the historical events and changes, and the town is currently recovering from the Serb destruction in the last liberation war fought for the historical rights to independence.