1. Kalemegdan Fortress
Belgrade's oldest structure sits right where the Sava meets the Danube, and it has been fought over, rebuilt, and repurposed so many times that the layers of stone read like a compressed history of Europe. Roman foundations, Byzantine walls, Ottoman gates, Austrian bastions. Over 2 million people walk through every year, making it the most visited spot in the city, and the entrance is free. The fortress grounds double as Belgrade's favorite park, so you will see families, joggers, couples on benches, and chess players at stone tables all mixed in with the ancient ruins. Inside the complex, the Ruzica Church has a chandelier made from spent bullet casings and swords, a detail that perfectly captures Belgrade's relationship with its own past. The Victor monument (Pobednik) at the edge of the cliff has become the city's symbol, standing with a sword and a falcon, staring out over the confluence. From this same viewpoint, you can spot Great War Island directly below and Zemun across the water. It is the one thing in Belgrade that everyone does, and for once, the crowds are justified.