The Kalemegdan complex is the main historical place in Belgrade. In addition to the fortress itself, there is a zoo, an amusement park for children, playgrounds, a bandstand, a dinosaur park, lots of fountains and places to walk. Stunning views from the fortress of the Danube and Sava rivers (the fortress is located at the confluence of these rivers). When staying in Belgrade, it is difficult not to visit Kalemegdan. I highly recommend it.
The entrance to the fortress is free, souvenirs are sold on the territory, there are museums and a dinosaur park, an exhibition of military equipment, and you can also buy something to eat. The fortress offers stunning views of the city. I recommend a visit.