Evidence of early knowledge about Belgrade's geographical location comes from a variety of ancient myths and legends. The ridge overlooking the confluence of the Sava and Danube rivers, for example, has been identified as one of the places in the story of Jason and the Argonauts. In the time of antiquity, too, the area was populated by Paleo-Balkan tribes, including the Thracians and the Dacians, who ruled much of Belgrade's surroundings. Specifically, Belgrade was at one point inhabited by the Thraco-Dacian tribe Singi; following Celtic invasion in 279 BC, the Scordisci wrested the city from their hands, naming it ''Singidūn'' (''d|ūn'', fortress). In 34–33 BC, the Roman army reached Belgrade. It became the romanised ''Singidunum'' in the 1st century AD and, by the mid-2nd century, the city was proclaimed a ''municipium'' by the Roman authorities, evolving into a full-fledged ''colonia'' (the highest city class) by the end of the century. While the first Christian Emperor of Rome —Constantine I, also known as Constantine the Great—was born in the territory of Naissus to the city's south, Roman Christianity's champion, Flavius Iovianus (Jovian), was born in Singidunum. Jovian reestablished Christianity as the official religion of the Roman Empire, ending the brief revival of traditional Roman religions under his predecessor Julian the Apostate. In 395 AD, the site passed to the Eastern Roman or Byzantine Empire. Across the Sava from Singidunum was the Celtic city of Taurunum (Zemun); the two were connected with a bridge throughout Roman and Byzantine times.