Karlovac was built from scratch in 1579 in order to strengthen Austria's southern defences against Ottoman encroaches. The establishment of a new city-fortress was a part of the deal between the Protestant nobility of Inner Austria and the archduke Charles II of Austria, in exchange for their religious freedom the nobility agreed to finance the building of a new fortress against the Ottoman Empire. It was founded as a six-pointed star fortress built on the Zrinski estate near the old town of Dubovac at the confluence of the Kupa and Korana rivers. As the city later expanded, the urban area reached as far as the Mre�nica and Dobra rivers. The unique star shape can still be seen around the town. It was originally known as Karlstadt ("Charles's Town" in German), after Archduke Charles II of Austria, upon whose orders construction began on July 13, 1579. The architect of the city was Matija Gambon.
The first church (of the Holy Trinity) was built in the central square in 1580, but all of the city buildings burned down in the fire of 1594. The forces of the Ottoman Empire laid siege to Karlovac seven times, the last time in 1672, but failed to occupy it. The plague epidemic of 1773 was also a notable threat to the city, decimating almost half the population at the time.
As a military outpost of the Habsburg Monarchy, Karlovac was the site of the trial and execution of the best-known leader of the rebel Uskoks from the coastal fort of Senj, Ivan Vlatkovi?. He was executed in Karlovac on 3 July 1612 as an example to his troops who were creating difficulties for the Habsurgs by their piracy against Venetian shipping on the Adriatic Sea, and by marauding raids into the Ottoman hinterland. In 1615 their piracy went so far as creating an open war between Venice and Austria. When the Treaty of Madrid[disambiguation needed] was concluded in 1617, bringing an end to the war between Venice and the Habsburgs, under the terms of the treaty the Uskok families were
