In the new era, during the Roman period, there was a fort here connecting today's Petrovaradin and Sremska Kamenica, called Kusum. In the Byzantine era, a fortress named Petrikon stood in this place. In the 8th century, Petrovaradin was part of the Frankish state, and from the 9th century it belonged to the medieval state of the Hungarians. In 1237, Hungarian King Bela IV named the fortress Petrvarad (Peter’s Castle), which later transformed into Petrovaradin. In 1526, the fortress was conquered by the Ottomans, and from 1691 it returned under the rule of the Austrian monarchy. The following year, construction of a new fortification began, which lasted more than 90 years and spanned the reigns of five Austrian rulers: Leopold I, Joseph I, Charles VI, Maria Theresa, and Joseph II. The fortress’s construction was initiated by the Austrian military command with financial assistance from the Pope to prevent future Turkish invasions.

The construction was started by the Italian Count Aloysius Ferdinando Luigi Marsili, a colonel-engineer of the Austrian army. First, the foundations and the most important above-ground structures were built, then the bastions, the outpost fortification on the other bank of the Danube, and a small island fortification. In the final stage, from the mid-18th century, the main buildings were erected — barracks, the Arsenal, a tower with large clocks, a water supply system, and the underground parts of the fortress — tunnels, galleries, and corridors on four levels.

Today, the Petrovaradin Fortress, or the "Gibraltar on the Danube," is one of the best-preserved fortifications in Europe and represents an excellent example of military architecture. The fort was built between 1692 and 1780, covering an area of 112 hectares, featuring 10 entrance gates, 12,000 firing holes for shooters, and platforms for 400 cannons.

Ultimately, the fort was constructed between 1692 and 1780, covering 112 hectares, with 10 entrance gates, 12,000 firing holes for shooters, and platforms for 400 cannons. The "Hungarian Gibraltar" was a key fortification on the Austro-Turkish border, one of the best-preserved fortifications in Europe, and an excellent example of military architecture.

The fortress’s construction, following Vauban’s designs, was overseen by Austrian generals Kaiserfeldt and Marsili. In 1694, the Turks unsuccessfully attempted to capture the unfinished fortress. The main event of the next Austro-Turkish war was the Battle of Petrovaradin (1716), won by Eugene of Savoy.

As the border moved further south, the fortress’s military importance declined, but its fortifications continued to be improved. By 1776, when the main construction works were completed, the Petrovaradin fortification system included 16 km of underground passages.

Many legends are associated with the fortress’s construction. For example, it is said that live cats were used when laying the foundations and walls. It was believed that cats have nine lives, and to make the fortress durable and indestructible, cats were walled in alive during its construction. Other legends mostly concern the underground parts of the fortress, hidden treasures in its depths, wandering ghosts of deceased builders, tunnels leading to the center of Novi Sad, and to unknown places...

During World War I, the Petrovaradin Fortress was in Austrian hands; in early 1918, it passed to the Kingdom of Serbia, and at the start of World War II, it became part of the NDH (Independent State of Croatia) and was liberated on October 23, 1944.

The lower part of the Petrovaradin Fortress developed in the 18th century into a military-residential complex with numerous squares, buildings, a monastery, as well as massive gates and bastions with defensive character. The Lower Fort is designed according to the principles of military Baroque architecture and contains several very important historical sites. The first is a monumental complex consisting of the former Jesuit monastery and the Roman Catholic parish Church of St. George, built in Baroque style. Then there is the Serbian Orthodox Church of St. Apostle Paul, as well as one of the fortress’s most important buildings — the Belgrade Gate. The Upper Fort is dominated by the Clock Tower, which, besides its appearance, is also interesting because the large hand shows the hours, and the small hand shows the minutes. The clock is also known as the "Drunken Clock" because it runs slow in cold winter weather and fast in hot summer weather. The clock is wound manually every day.

During the revolutionary upheavals of 1848–1849, insurgents barricaded themselves inside the fortress. After that, it was no longer used for military purposes.

In 1914, the fortress temporarily held senior non-commissioned officer of the Austro-Hungarian army Josip Broz, the future president of Yugoslavia. He was imprisoned there on charges of inciting rebellion but was soon acquitted and released.

The Petrovaradin Fortress is a popular tourist attraction. Its hallmark is the clock tower with "incorrect" clocks, whose minute hand is slightly shorter than the hour hand. Since 2001, the international music festival Exit has been held on the fortress grounds. Today, about a hundred art studios are located here, along with restaurants, cafes, museums, and galleries.

Sources:

https://ru.wikipedia.org/wiki/Петроварадинская_крепость

https://rg.ru/2021/03/15/petrovaradinskaia-krepost-iavliaetsia-glavnoj-znamenitostiu-novi-sad.html

https://dzen.ru/a/X_HD5_kGsWhylmie