[5] From 1579 onwards, he had the City of Karlovac (Karlstadt), named in honour of Archduke Charles II,[6] built in order to strengthen the Habsburg southern defences against Ottoman encroaches.

[7] The establishment of the new city was a part of the deal between the Protestant nobility of Inner Austria and the archduke: in exchange for their religious freedom the nobles agreed to finance the building of a new fortress against the Ottoman Empire.

The six-pointed star fortress was built on the Zrinski estate near the old town of Dubovac at the confluence of the Kupa and Korana rivers, on terrain exposed to flooding and disease from unhealthy water, with the intent to hamper the Turkish advance.

By his two marriages, he could provide substantial financial resources to acquire the castles of Hochosterwitz and Wernberg by purchase;[11][12] he had both rebuilt in a lavish Renaissance style.

He also supervised the redevelopment of Klagenfurt as the new Carinthian capital and had a city palace built in Villach about 1570, which however was destroyed in World War II.