There is little trace left of this era of the city's history, except for burial sites, a few of which are the tribe's warriors that contain valuable artifacts. A residual artifact leftover from the tribes that secured the area is the Celtic spiritual influence that was taken up and woven into the Roman classical culture of the city.
The Romans first began to conquer lands surrounding Singidun during the first 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, Dardanians, 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 AD 6, 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.
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, Gepidaes, 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:
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 was lost again it 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. 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.