Construction of Sisak fortress, at the confluence of rivers Kupa and Sava, started in 1544. and was financed by Zagreb Archdiocese. Its purpose was the defense from invading Ottoman forces.
On 22nd June 1593, the field in front of the fortress became the scene of one of the most important battles in Croatian history.
Battle of Sisak
Hasan Pasha was known for his fighting spirit, and he didn't care much abouth the truce that was concluded with envoys of German Emperor and Croatian-Hungarian king, Rudolf II. On the contrary, in 1592 he seizes the Croatian fortress Bihać (today city in Bosnia), besieges the fortress of Sisak for the second time, but without success, and builds a fort named 'Yeni Hisar' at the confluence of rivers Kupa and Petrinjčica (today city of Petrinja, 12 km away from Sisak).
In mid-June 1593, with an army of about 12000 men, Hasan Pasha reaches the fortress of Sisak and besieges it for the third time. At the same time, the Christian army, 5000 men strong, assembled near Zagreb. In the joint Christian army of the Holy Roman Empire, Croats were commanded by Croatian ban Tomo Bakač Erdödy (Hungarian: Tamás Erdődy), Slovenes were commanded by general Andreas von Auersperg (Slovene: Andrej Turjaški, Croatian: Andrija Auersperg), and supreme commander was Styrian general Ruprecht von Eggenberg.
On 22nd June 1593, Christian army arrives to Sisak and clashes with the Ottomans. Turks, suppressed by concentrated fire of the Christian army marksman (musketeers and arquebusiers), panicked and started a chaotic retreat, until they were penned in a narrow space between two rivers. The bulk of the army with all the commanders, including Hasan Pasha Predojević, are said to have been slaughtered or drowned in the Kupa river.
It is estimated that over 8000 Turks were killed, while only 40–50 casualties were counted on Christian side. This battle is considered to be a turning point in a history of clashes along the Croat-Ottoman border, that lasted for over a 100 years until then. Defeat brought the expansion of Ottoman Empire into Central Europe to a temporary halt, and balanced the military power of the conflicting parties.