Sirmium

Sirmium was a city in the Roman province of Pannonia, located on the Sava river, on the site of modern Sremska Mitrovica in the Vojvodina autonomous province of Serbia. First mentioned in the 4th century BC and originally inhabited by Illyrians and Celts, it was conquered by the Romans in the 1st century BC and subsequently became the capital of the Roman province of Pannonia Inferior. In 293 AD, Sirmium was proclaimed one of the four capitals of the Roman Empire. It was also the capital of the Praetorian prefecture of Illyricum and of Pannonia Secunda. The site is protected as an archaeological Site of Exceptional Importance. The modern region of Syrmia was named after the city.

Sirmium purportedly had 100,000 inhabitants and was one of the largest cities of its time. Colin McEvedy, whose estimates for ancient cities are much lower than the general consensus, put the population at only 7,000, based on the size of the archaeological site.

Source: Wikipedia