The Austrians built Karlovac from scratch in 1579 in order to strengthen their southern defences against Ottoman encroachments. 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 Mrežnica and Dobra rivers. The star shape can still be seen around the town. It was originally known as Karlstadt ("Charles's Town" in German), after the ruling family, upon whose orders construction began on July 13, 1579. The architect of the city was Matija Gambon, whilst work on the new fortress was supervised by George Khevenhüller. It was intentionally built on terrain exposed to flooding and disease from unhealthy water, with the intent to hamper the Turkish advance.