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Towering over the river on a 40m-high volcanic slab, this mighty citadel, considered Europe's second-biggest fortress (and one of its best preserved), is aptly nicknamed 'Gibraltar on the Danube'. Constructed using slave labour between 1692 and 1780, its dungeons have held notable prisoners including Karađorđe (leader of the first Serbian uprising against the Turks and founder of a royal dynasty) and Yugoslav president Tito.
Have a good gawk at the iconic clock tower: the size of the minute and hour hands are reversed so far-flung fisherfolk can tell the time. Within the citadel walls, a museum offers insight into the site's history; it can also arrange tours (in English; 3500RSD) of Petrovaradin's 16km of creepy, but cool, unlit underground tunnels (katakombe). Petrovaradin hosts Novi Sad's wildly popular EXIT Festival each July.