Emperor August first invaded Illyricum in 35 BC, but Romans stabilized themselves in Sirmium only with Emperor Tiberius during his Pannonia Wars around 10 BC. Roman castrum was built on the River Sava and this is probably the reason why the city that later grew around castrum was called Sirmium which means: to flow, flowing river or wetland. As such, Ancient Roman Sirmium used to be a part of very complex system of Roman strongholds allocated down Sava and Danube rivers to prevent potential Barbaric attacks coming from the North. By 294 AD, Sirmium became so important that Emperor Diocletian proclaimed it one of four capitals in so called “Tetrarchy governing system”. Therefore, Sirmium used to serve as temporary residence for emperors: Domitian, Marcus Aurelius, Septimus Severus, Maximinus Thrax, Claudius II Gothicus, Probus, Diocletian and the infamous usurpers Ingenuus and Regalian. Five Roman Emperors were born in or near Sirmium – Trajan Decius, Aurelian, Probus, Maximianus Herculius and Gratian.
As far as excavations are concerned, Emperor’s Palace, numerous public buildings and multistoried insulaes are uncovered. Roman Hippodrome is still buried under modern Sremska Mitrovica city center. Sirmium was protected by a circuit of strong defensive walls and aquaducg from the Vranješ spring in Fruška Gora Mountain supplied the city with fresh water. Sirmium streets were paved, flanked by porticoes and drained with sewers. Coins were struck in the Imperial mint in Sirmium and workshops produced various objects in precious metals, glass and pottery. Bricks were also manufactured in Sirmium. The well-known Roman historian of the 4th century, Ammianus Marcellinus described Sirmium as „the glorious and populous mother of cities“.