The heart of the city lies within the walls of Roman emperor Diocletian's retirement palace, which was built in the 3rd century AD. Diocletian, born in the nearby Roman settlement of Salona in AD 245, achieved a brilliant career as a soldier and became emperor at the age of 40. In 295 he ordered this vast palace to be built in his native Dalmatia, and when it was completed he stepped down from the throne and retired to his beloved homeland. Upon his death, he was laid to rest in an octagonal mausoleum, around which Split's magnificent cathedral was built.In 615, when Salona was sacked by barbarian tribes, those fortunate enough to escape found refuge within the stout palace walls and divided up the vast imperial apartments into more modest living quarters. Thus, the palace developed into an urban center, and by the 11th century the settlement had expanded beyond the ancient walls.Under the rule of Venice (1420–1797), Split—as a gateway to the Balkan interior—became one of the Adriatic's main trading ports, and the city's splendid Renaissance palaces bear witness to the affluence of those times. When the Habsburgs took control during the 19th century, an overland connection to Central Europe was established by the construction of the Split–Zagreb–Vienna railway line.After World War II, the Tito years saw a period of rapid urban expansion: industrialization accelerated and the suburbs extended to accommodate high-rise apartment blocks. Today the historic center of Split is included on UNESCO's list of World Heritage Sites.