I have learnt that the fortress Kalemegdan has a very long history, going back at least to the castrum of Roman times; destroyed several times by successive waves of invaders, and was rebuilt as a castle by the Byzantines in the 12th century.
Under the Serb Despot Stefan Lazarevic, son of the king Lazar, Belgrade became the capital of the Serbian kingdom; the fortress was strengthened, and the Despot's palace was built within the old castle.
A medieval town grew up within the walls of the lower fort.