Kalemegdan is Belgrade’s central park and fortress complex lying on a hill overlooking the Sava and Danube confluence, on the eastern side of the river Sava. This has been the site of the ancient Roman city of Singidunum, the medieval and Turkish era Belgrade and was converted into a park in the middle of 19th century. It is home to several galleries and museums, restaurants, sports courts, and the Belgrade Zoo.
https://www.tob.rs/en/what-to-see/belgrade-attractions/belgrade-fortress