Singidunum ( today Belgrade ) was founded by the Romans in the 1st century A.D. It had a strategic importance for the Roman Empire of which the borders were on the river Danube at the time. Singidunum was defended by the Roman legion IV Flaviae which built a fortified camp on a hill at the confluence of the rivers Danube and Sava.
In the period between 378 A.D. and 441 A.D. the Roman camp was being destroyed in the invasions by the Goths and the Huns.
The Byzantine Emperor Justinian rebuilt the Fortress around 535 A.D. In the following centuries it suffered continuos destruction under the Avar sieges. The name Belgrade, which in most Slavic languages means a white town
or a white Fortress, was first mentioned in 878 A.D. after the Slavs had settled in the Balkans at the end of the 7th century.
The Fortress remained a Byzantine stronghold until the 12th century when it fell in the hands of a newly emerging Serbian state. At the end of the 13th century Belgrade became its cultural and commercial center. 132 years after the  Battle of Kosovo in 1521 the Fortress, like most parts of the Serbian state, was conquered by the Turks and remained under the rule of the Ottoman Empire until the year 1867 when the Turks withdrew from Belgrade.
The Fortress suffered further damages during the First and the Second world wars. After almost two millennia of continuous sieges, battles and conquests the Fortress is today known as the Kalemegdan fortress. The name derives from the Arabic and Turkish languages and signifies  the Fortress amidst battlefields.