In c. 600 B.C. the Thracian-Cimmerian and Scythian tribes moved across this area, while the Celtic tribes crossed this territory in the III century B.C. The founding of Singidunum is attributed to the Celtic tribe, the Scordiscs. As a fortified settlement, Singidunum was mentioned for the first time in 279 B.C. The first part of the word - Singi - means "round" and dunum means "fortress" or "town". The name of the settlement was preserved throughout the Roman rule. The Romans conquered Belgrade in the beginning of the I century A.D. and it has been under their rule for full four centuries. Singidunum was most prosperous in 86 A.D., when the IV Legion of Flavius arrived. As an important Roman military camp, Singidunum gained municipal rights in the II century A.D. during the rule of emperor Hadrian. Its military importance became even higher in the III century. 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.
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, Belgrade suffered frequent attacks and destructions. The attacks coming from the north, across Pannonia, the Danube and Sava, were so hard that even Singidunum, an important military stronghold, could not resist them. 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, already in 488, it became a Byzantine town again. Byzantine emperor Justinian I rebuilt Singidunum in 535, restoring the fortress and city to its former military importance. At the end of the VI century, while the Byzantines were occupied with wars in Africa and Asia, the Mongol tribes of Avars appeared in front of the walls of Singidunum, and after them came the first groups of the Slavs. The name Singidunum disappeared after this barbarian invasion and the destruction of the town and it has never appeared again in the whole history afterwards.
The city would re-emerge later, mentioned as Beograd, a Slavic word meaning "white fortress" (due to the color of the stone it was built from), in a letter written on April 16, 878 by Pope John VIII to Bulgarian prince Boris I Mihail. With its new name, Beograd, would eventually be restored to the same strategic significance it had held throughout its history, but never again would it be mentioned as Singidunum. Several centuries after the first mentioning of Belgrade as a Slavic town, various armies and conquerors control it by turns. The Franks were the first to reach Belgrade and destroy the Avars under the command of Charles the Great. The rule of the Franks was replaced by Bulgarians, and they gave place to Hungarians. Already in 1018, it once again became a border stronghold of the Byzantine Empire. During the XI and XII centuries, the rival forces of Hungary, Byzantine Empire and Bulgaria fought for it. After the Crusades of 1096 and 1147, 190,000 people pass through Belgrade in 1189, led by Frederick Barbarossa. This leader of crusaders saw Belgrade in ruins.