Singidunum Singidunum ( , from a Celtic ' ) is the name for the ancient city in Serbia which became Belgrade , the capital of Serbia . It was recorded that a Celtic tribe , the Scordisci , settled the area in the 3rd century BC following the Gallic invasion of the Balkans . The Roman Empire conquered the area in 75 BC and later garrisoned the Roman Legio IV Flavia Felix in 86 AD . It was the birthplace to the Roman Emperor Jovian . Belgrade has arisen from its ashes 38 times . Foundation . The first evidence of primitive fortification came later in the 3rd century BC , with the settlement of the Scordisci who picked the strategic hilltop at the meeting of the two rivers as the basis for their habitation . The name " Singidun " is first attested in 279 BC . The name has Celtic " dūn ( on ) " " enclosure , fortress " as its second element , but possibly also Slavic word for " blue " ( " siniy " in Serbian ) , meaning " blue Danube " ( " siniy Dunav " in Serbian ) , after the river Danube . For " singi - " there are several theories , the two most widely circulated being that it is a Celtic word for " circle " , hence " round fort " , or it could be named after the Sings , a Thracian tribe that occupied the area prior to the arrival of the Scordisci . Another possibility is that it is a composite name the first part of which ( Sin - gi ) means " Old prayer " ( " sean gui " in Gaelic ) , implying that this was originally site of Celtic religious significance , in addition to becoming a fortress ( dun ) . This would jive with the ancient Celtic burial practice remnants there . There is only limited archaeological evidence of the city ' s foundational period , including some burial sites with grave goods . Roman era . The Romans first began to conquer lands surrounding " Singidun " during the 1st century BC . In 75 BC , Gaius " Quintus " Scribonius Curio , the proconsul of Macedonia , invaded the Balkan interior as far as the Danube , in an effort to drive out the Scordisci , Dardani ans , Dacians and other tribes . The Romans had victories during these campaigns , but only stayed briefly , leaving the area outside of Roman control . Thus , very little is known about these operations or when the area was organized into the province of Moesia . It wasn ' t until the rule of Octavian , when Marcus Licinius Crassus , the grandson of the Caesarian Triumvir and then proconsul of Macedonia , finally stabilized the region with a campaign beginning in 29 BC Moesia was formally organized into a province some time before 6 AD , when the first mention of its governor , Caecina Severus , is made . " Singidun " was Romanized to " Singidunum " . It became one of the primary settlements of Moesia , situated between Sirmium ( modern Sremska Mitrovica ) and Viminacium ( modern Kostolac ) , both of which overshadowed Singidunum in significance , and just across the Sava River from Taurunum ( modern Zemun ) in Pannonia . Singidunum became an important and strategic position along the " Via Militaris " , an important Roman road connecting fortresses and settlements along the Danubian " limes " , or border . Singidunum reached its height with the arrival of Legio IV " Flavia Felix " in 86 AD . The legion set up as a square - shaped " castrum " ( fort ) , which occupied Upper Town of today ' s Kalemegdan . At first , the fortress was set up as earthen bulwarks , but soon after , it was fortified with stone , the remains of which can be seen today near the northeastern corner of the acropolis . The legion also constructed a bridge over the Sava , connecting Singidunum with Taurunum . The 6 , 000 - strong legion became a major military asset against the continuous threat of the Dacians just across the Danube . Another step the Romans took to help strengthen Singidunum was the settlement of its legion veterans next to the fortress . In time , a large settlement grew out from around the " castrum " . The town took on a rectlinear construction , with its streets meeting at right angles . The grid structure can be seen in today ' s Belgrade with the orientation of the streets Uzun Mirkova , Dušanova , and Kralja Petra I . Studentski Trg ( Students ' Square ) was a Roman " forum " , bordered by " thermae " ( a public bath complex whose remains were discovered during the 1970s ) and also preserves the orientation the Romans gave Singidunum . Other remnants of Roman material culture such as tombs , monuments , sculptures , ceramics , and coins have been found villages and towns surrounding Belgrade . Hadrian granted Singidunum the rights of " municipium " during the mid 2nd century . Singidunum later outgrew this status and became a full - fledged colony . The Roman Emperor Jovian who reestablished Christianity as the official religion of the Roman Empire was born in Singidunum in 332 . Singidunum and Moesia experienced a peaceful period , but that was not to last , due to the growing turmoil not only from outside the Roman Empire , but also from within . The Roman Empire began to decline at the end 3rd century . The province of Dacia , established by several successful and lengthy campaigns by Trajan , began to collapse under pressure from the invading Goths in 256 . By 270 , Aurelian , faced with the sudden loss of many provinces and major damage done by invading tribes , abandoned Dacia altogether . Singidunum found itself once again on the " limes " of the fading Empire , one of the last major strongholds to survive mounting danger from the invading barbarian tribes . Byzantine rule and Migration period . In 395 , upon the death of Theodosius I , the Roman Empire was split into two , with Singidunum lying on the northwestern border of the Eastern Roman Empire ( later to become the Byzantine Empire ) . Moesia and Illyricum suffered devastating raids by the successive invasions of the Huns , Ostrogoths , Gepid s , Sarmatians , Avars , and Slavs . Singidunum fell to the Huns in 441 , who razed the city and fortress , selling its Roman inhabitants into indentured servitude . Over the next two hundred years , the city passed hands several times : the Romans reclaimed the city after the fall of the Hun confederation in 454 , but the Sarmatians conquered the city shortly thereafter . In 470 the Ostrogoths seized the city around , expelling the Sarmatians . The city was later invaded by Gepids ( 488 ) , but the Ostrogoths recaptured it in 504 . Six years later the Eastern Roman Empire reclaimed the city according to a peace treaty . Byzantine emperor Justinian I rebuilt Singidunum in 535 , restoring the fortress and city to its former military importance . The city saw a brief peaceful period of about fifty years , but was then sacked with the arrival of the Avars in 584 . During Maurice ' s Balkan campaigns , Singidunum served as a base of operations , but it was lost again in the early half of the 7th century when the Avars sacked and burned Singidunum to the ground . Around 630 , the Slavs settled in the area and in Singidunum , coordinated by a Roman fortress commander . By this time , however , the city had lost its importance as a border fortification and was largely ignored by the Slavs , who dominated the area . Belgrade . The Slavic " Beligrad " , the " white city " ( named for the color of the stone it was built from ) , had been established by the 9th century on the site of Singidunum , within the First Bulgarian Empire . It is first mentioned in a letter written on 16 April 878 by Pope John VIII to Bulgarian prince Boris I Mihail . With its new name , Beograd would eventually be restored to its earlier strategic significance .