In ancient Roman times, Serbia used to be known as Mesia province. Roman legions serving in Serbia managed to build complex system of border strongholds connected by Via Militaris or Military road. This system followed Sava and Danube rivers, it was seen as impenetrable line of defense by the Barbarians, and it was called limes in ancient Latin. Viminacium Castrum (Kladovo today) was bigger and more important than Sirmium (Sremska Mitrovica) and Singidunum (Belgrade). Seventeen Roman emperors, in total, were born in Serbia among them Constantine the Great, founder of Constantinople.
Ancient Celtic tribes were chased away from the Balkans by the Ancient Romans, who created complex border defensive system on rivers Sava and Danube called Limes, as Viminacium (Kladovo), Singidunum (Belgrade), and Sirmium (Sremska Mitrovica) acted as the biggest Roman castrums/cities. Serbia was then called Mesia Province in Latin.