Файл:Kalemegdan "Lower Town".jpg

Kalemegdan is the core and the oldest section of the urban area of Belgrade and for centuries the city population was concentrated only within the walls of the fortress, thus the history of the fortress, until most recent history, equals the history of Belgrade itself. First settlement was founded in the 3rd century BC by the Celtic tribe of Scordisci. The city-fortress was later conquered by the Romans, became known as Singidunum and became a part of "the military frontier", where the Roman Empire bordered "barbaric Central Europe". Singidunum was defended by the Roman legion IV Flaviae which built a fortified camp on a hill at the confluence of the rivers the Danube and the Sava.

City’s history is long and complicated, more about Kalemegdan and Singidunum can be read on wikipedia. After almost two millennia of continuous sieges, battles and conquests the fortress is today known as the Kalemegdan fortress. The name Kalemegdan derives from two Turkish words, kale (fortress) and megdan (battleground) (literally, "battlefield fortress").

Source: en.wikipedia.org/wiki/Kalemegdan