The first coins minted by a Celtic tribe were imitations of Philip's tetradrachms in silver made by the Scordisci around Belgrade in the first half of the 1st century BC. The Serbian capital is one of the oldest towns in Europe. The remains of its long and turbulent history can still be seen on a hill overlooking the mouth of two rivers, the Sava and Danube. That is the site of the fortress of Belgrade which is actually layer upon layer of a different fortifications which were built, torn down and subsequently rebuilt over hundreds of years. The present-day archaeological and historical site is surrounded by the most attractive city park, bearing the old Turkish name of this place – Kalemegdan.

The area around the Sava and the Danube has been continuously inhabited since the paleolithic period. Remains of human bones and skulls of Neanderthals found in the stone-pit date back to the early Stone Age. Remains of the late Stone Age culture have been found in and around the Sava and Dunav confluence.

Around 600 BC the Thracian-Cimmerian and Scythian tribes moved across this area, while the Celtic tribes crossed this territory in the 3rd century BC. The founding of Singidunum is attributed to the Celtic tribe, the Scordisci. As a fortified settlement, Singidunum was mentioned for the first time in 279 BC. The first part of the word - Singi - means "round" and dunum means "fortress" or "town". It is possible that the name originated from the name of the Thracian tribe, the Sings, which was settled on this area when the Celts came. There are almost no traces of Singidunum left except the burials, some of which contain valuable artifacts from the warriors of the Scordiscan tribe. Celtic cultural influences were woven into the spiritual culture of the Singidunum inhabitants,later to be mixed with Roman classical cultural elements.

The Romans conquered Belgrade in the beginning of the 1st century AD and it was under their rule for 400 years. The soldiers of the Moesian legions made the first Roman garrison in Singidunum. Singidunum was most prosperous in 86 AD, when the Fourth Legion of Flavius arrived. The first stone fortress was constructed. Fragments of that "castrum" (fort) show that it was square-shaped and covered the area of today's Upper Town. During its development, Singidunum outgrew its status of a "municipium" and become a colony of the Roman citizens. As an important Roman military camp, Singidunum gained municipal rights in the second century AD during the rule of emperor Hadrian. Its military importance became even higher in the third century, when the emperor Aurelian left Dacia and Upper Moesia gained new borders along the right bank of the Danube. In that period, Singidunum was the center of the Christian diocese. Some time later, it was the place of birth of the Roman emperor Flavius Jovianus.

Between Singidunum and Taurunum, there was a bridge over the Sava, which connected the two towns and which was a part of one of the most important Roman roads. Thus, Singidunum became an important crossroad for the Roman provinces of Moesia, Dacia, Pannonia and Dalmatia. The military road Via Militaris, which went from the west to the east through Sirmium (Sremska Mitrovica), Singidunum and Viminacium (Kostolac) to Byzantium, was protected by forts. These forts also existed in the area of today's Belgrade.

After the division of the Roman Empire into the Western Roman Empire and Eastern Roman Empire in 395, Singidunum became a border town of the Byzantine Empire. This new position of the town determined its later fate, for it became not only a linking point of various cultural influences, but, before all, a communication and strategic key of the Byzantine Empire.

The disintegration of the Roman Empire was followed by invasion of barbarian peoples: Eastern Goths, Gepidaes, Sarmatians, Avars, Slavs and others. Because of its advanced position at the border - limes, Belgrade suffered frequent attacks and destruction. The attacks coming from the north across Pannonia, the Danube and Sava, were so hard that even Singidunum, an important military stronghold, could not stand. The Huns captured it and completely destroyed it in 441. Singidunum lost its Roman inhabitants then. After the fall of the Huns, the town became a part of the Byzantine Empire once again in 454, but it was soon conquered by the Sarmatians, and later the Eastern Goths. However, in 488, it became a Byzantine town again.