Petrovaradin Fortress (Serbian: Петроварадинска тврђава, Petrovaradinska tvrđava, pronounced [pɛtrɔʋarǎdiːnskaː tʋř̩dʑaʋa]; Hungarian: Péterváradi vár), nicknamed "Gibraltar on/of the Danube",[1][2][3] is a fortress in the town of Petrovaradin, itself part of the City of Novi Sad, Serbia. It is located on the right bank of the Danube river. The cornerstone of the present-day southern part of the fortress was laid on 18 October 1692 by Charles Eugène de Croÿ. Petrovaradin Fortress has many tunnels as well as 16 kilometres (9.9 mi) of uncollapsed underground countermine system. In 1991 Petrovaradin Fortress was added to Spatial Cultural-Historical Units of Great Importance list of the Republic of Serbia. https://en.wikipedia.org/wiki/Petrovaradin_Fortress