After Diocletinan's death, the palace remained an imperial possession, and was probably used by members of the imperial families at least until A.D. 480. The Notitia Dignitatum mentions that textile production was carried on there. In the fifth century a cross was carved on the architrave of the west gate over a bas relief of Victory. This is the earliest evidence of Christianity within the Palace.
The final transformation of the Palace into a town took place in the seventh century. When Salona was destroyed by an invasion of Avars and Slavs shortly after A.D. 612, some of the survivors took refuge in Split. According to a thirteenth century writer, only the richer refugees built houses; others settled in the towers and substructures of the old palace.
The new residents brought some of their old institutions with them. The most important of these was the ecclesiastical organization, which was reorganized and revived by the archbishop John of Ravenna. His program caused significant adaptations in the Palace. He turned Diocletian's mausoleum into a Christian church. Probably the Temple of Jupiter became a baptistery at the same time. These changes can be taken as inaugurating the early medieval period in architecture and art, which is generally considered to last until the end of the eleventh century. Other Roman buildings adapted for the needs of the new inhabitants during the medieval period included St. Martin's chapel in the Roman sentries' gallery above the north gate, and another chapel above the west gate.
During most of the first period of urban life, Split was under Byzantine administration, while the surrounding area was settled by Croatian Slavs who were ruled by their native princes, initially the vassals of Frankish kings. The Croatian rulers were developing a state which had the social structure of a tribal community. The existence of this Slavic population outside must have had its influence on the ethnic composition of Split itself. We have little evidence about the nature or rate of Croatian penetration into the town. In the tenth century Constantine Porphyrogenitos said that the inhabitants of Split and other Dalmatian coastal cities were still called Romani. Some Croats had apparently already settled there, however, because a sarcophagus inscription from the same century commemorates a man born in Split, whose father had a Slavic name, and who became Archbishop. From the eleventh century on, city records mention citizens whose names are Slavic in origin. By the thirteenth century, many members of the town council were Slavs.
Several times in the tenth and the eleventh centuries Split came under the rule of Croatian Kings. At the beginning of the twelfth century it, like other Dalmatian towns, became a free commune under the direct authority of a Hungaro-Croatian king. This political position benefited the economic and architectural development of Split. The extent of its urban territory doubled, spreading out west of the palace. The new process radically changed the original town plan by reducing the width of ancient streets and lining them with Romanesque and Gothic houses.
The ancient Peristyle, probably both the religious and the municipal center of the early medieval town, by the end of the Middle Ages retained only the role of an ecclesiastical square, architecturally dominated by the Romanesque bell tower of the Cathedral. The municipal center had moved to the new part of Split west of the Palace.
At the beginning of the fifteenth century, the medieval free commune was replaced by a Venetian administration which lasted until the end of the eighteenth century. Noblemen built mansions in the late Gothic and early Renaissance styles, the finest of which are whose designed by Juraj Dalmatinac and his school. The fortified building complex (Castello) southwest of the palace was one of the most outstanding buildings of the period.
The second stage of Venetian rule was characterized by artistic and architectural stagnancy caused primarily by frequent wars between Venice and Turkey. A few Baroque mansions of modest values were built, but the greatest visual impact was created by the strong polygonal fortifications which encompassed the whole town.
At the beginning of the nineteenth century, after a short period under French rule, the town fell under the Austrio-Hungarian empire (1813-1918). It then expanded far beyond its earlier boundaries and absorbed several rural settlements which had grown up around it during the Middle Ages. It was given an important role as an administrative, cultural and, in particular, economic center; factories were erected and the harbour obtained greater economic significance.
After World War I, the role of Split as a cultural, administrative and economic center became still greater, reflecting the town's expansion and large population influx. Following the Second World War, during which the town suffered from frequent bombing raids, the most dynamic development of Split began.