Petrovaradin Fortress, also known as the "Gibraltar of the Danube," is a fortress located on the right bank of the Danube River in Novi Sad, Serbia. The fortress was built in the 17th and 18th centuries by the Austrians to protect their southern border against Ottoman invasions.

The fortress consists of a series of walls, gates, and bastions spread over an area of 112 hectares. The most prominent feature of the fortress is the Clock Tower, which is 62 meters tall and offers stunning views of the surrounding area. The fortress also has a system of underground tunnels, which were used for storage, communication, and as a shelter during wartime.

Today, Petrovaradin Fortress is a popular tourist attraction and cultural center. It hosts various festivals and events throughout the year, including the famous Exit Festival, one of the largest music festivals in Europe. Visitors can explore the fortress on foot, or take a guided tour to learn more about its history and architecture.

In addition to its cultural and historical significance, Petrovaradin Fortress also serves as an important scientific and educational center. It houses the Faculty of Sciences of the University of Novi Sad, as well as several research institutes and laboratories.