A string of forts along the Una, centred around Gvozdansko and in possession of the Zrinski noble family, formed the main line of defense of Croatia since 1527.

The Ottoman army besieged Gvozdansko on 3 October, whose garrison numbered 300 soldiers and miners, under the command of Damjan Doktorović and three other Croatian captains.

By the final day of the siege, on 13 January, the entire Croatian garrison was dead and Ottoman forces entered the fort in the early morning.

[3] Gvozdansko was a rectangular fort in possession of the Zrinski family, located on the main road between the towns of Dvor and Glina.

In the space of six months, over half of Slavonia was under Ottoman control, from Valpovo to the fort of Jasenovac at the mouth of the Una into the Sava.

[9] Shortly after the fall of Dubica, the Croatian nobility met in Zagreb to evaluate their defensive capabilities, and pointed out the lack of military forces and financial difficulties.

[11] As Suleiman was busy with the war with Venice, the activities on the Ottoman–Croatian border calmed down until October 1539, when Zrinski had Katzianer executed for siding with Zápolya and negotiating the surrender of Kostajnica to the Ottomans.

Croatian Ban Petar Keglević sent an army to help Zrinski, but immediately called it off at the request of Queen Anne, in order not to violate the agreed armistice.

Two years later, Zrinski was named Ban of Croatia,[17] in place of Keglević, who was removed from office for obtaining possessions of his daughter's late husband.

[26] In 1552, as the peace treaty came to an end, the sanjak-beys of Bosnia, Herzegovina, and Klis received orders from Constantinople to mobilize their forces near Banja Luka.

Under the command of Ulama Bey of Požega, the Ottoman army captured two important fortified towns north of the Sava, Virovitica and Čazma, and came within 35 miles from Zagreb.

The conquest of the remaining Croatian territory between the Drava and Sava rivers would make it easier to carry out future attacks on the Archduchy of Austria.

[29] Ferdinand's proposal to abandon and destroy the forts of Gvozdansko, Zrin, Slunj, and several others in the Una Valley due to their vulnerability was rejected during the parliament's session.

[38] Activities on the Una frontier resumed in 1565, when the fort of Krupa surrendered to a force led by Mustafa Pasha Sokolović, who was appointed Bosnian sanjak-bey a year earlier.

[39] Ottoman Grand Vezir Sokollu Mehmed Pasha, who held the position from 1566 to 1579,[41] concluded that a halt was needed in their military activities.

[43] Emperor Maximilian II, who succeeded Ferdinand in 1564, sought no further confrontation with the Ottoman Empire and concluded an eight-year peace deal with the High Porte in February 1568.

Ottoman attacks largely targeted the strategically important fort of Hrastovica near Sisak, which would enable them to cross the Kupa.

[44] Extraordinary tax measures were introduced by the Croatian Parliament to strengthen the border forts, including Hrastovica,[45] which withheld two Ottoman assaults in 1568 and 1571.

[48] The Ottoman Empire concluded a peace treaty with Venice in March 1573, which enabled local sanjak-beys to direct more forces towards Croatia.

[50] Ferhad Bey Sokolović, who became the sanjak-bey of Bosnia in 1574,[51] and would become a pasha six years later,[52] led several offensives on border forts during 1575, including an unsuccessful attack on Bihać.

In September 1575, he defeated an army led by Herbard VIII von Auersperg, the commander of the Croatian Frontier, who was killed in the battle of Budački.

In the same month, Ferhad Bey's troops captured the fort of Bila Stina on the left bank of the Una,[50] located between Cazin and Ostrožac,[53] which enabled an Ottoman breakthrough towards the Korana, Mrežnica, and Kupa rivers.

The difficult situation in the kingdom was also affected by the unclear position of the banship, as both Alapić and Drašković asked the Emperor to relieve them of their duties.

The banship was then offered to captain Krsto Ungnad,[56] whose terms, regarding the salary and number of soldiers allocated to the ban, were rejected by Maximilian.

[52] Ferhad Bey's offensive resumed in September 1577, when he personally led an army to capture the remaining forts up to the Glina River, north of Cazin.

[60] One soldier managed to pass through Ottoman lines and reach the Croatian-held town of Steničnjak, where captain Johann Auersperg was stationed.

[60] A part of the Ottoman army was left to keep Gvozdansko under siege, while Ferhad Bey led the rest and attacked the remaining Croatian forts in the region.

Three major assaults on Gvozdansko were repelled on 10, 11, and 12 January 1578, leaving only 25–30 men still alive that held their positions on the last days of the siege.

When they breached the gates of the fort, they found only corpses of soldiers that died of wounds, hunger, thirst, or were frozen to death.

Ferhad Bey did not disturb the construction of Karlovac, and focused on the repairing of Gvozdansko, Zrin, and other recently captured forts.