Petrovarain Fortress (Serbian: Петроварадинска тврђава or Petrovaradinska tvrđava, Hungarian: Péterváradi vár) is a fortress on the Danube river, near Novi Sad (Hungarian: Újvidék) in the Serbian province of Vojvodina. The cornerstone of the present-day southern part of the fortress was laid on October 18, 1692, by Prince Croy. Petrovaradin Fortress has many underground passages as well (16 Km of underground countermine system).