Diocletian was a Roman emperor who ruled between AD 284 and 305 and was known for his reforms and persecution of Christians. He ordered the work on the palace to begin in 293 in readiness for his retirement from politics in 305. The palace faces the sea on its south side and its walls are 170 to 200 m (570 to 700 feet) long and 15 to 20 m (50 to 70 feet) high, and it encloses an area of 38,000 m² (9½ acres). This massive structure was long deserted when the first citizens of Split settled inside its walls. In 639, the interior was converted into a town by the citizens of Salona who escaped the destruction of their town by the Avars. Over the centuries, the city has spread out over the surrounding landscape, but even today the palace constitutes the inner core of the city, still inhabited, full of shops, markets, squares, with even a Christian cathedral (formerly Diocletian's mausoleum) inserted in the corridors and floors of the former palace. Although part of Byzantine Empire, the town had political autonomy.
The rise of the early Medieval Croatian state in neighbouring littoral (coastal cities) and the hinterland provoked in the following centuries Split developed a Croatian character, which can be seen in the architecture of churches in the city and surroundings, and which led to the unity of the Church with Split at the center in 928; it is important to mention that there was an important church synode, where a clerical jurisdiction over Croatia and relations of Latin-rite and Croat (slavic)-rite in church in Croatia were discussed.
At the beginning of the 12th century Split was led by nobility of Kingdom of Croatia-Hungary. The city however maintained independence, as in 1312 it issued statues and had currency of its own. The Venetian Republic took control of Split in 1420, when the population was Croatian, but the common language was also italian. The autonomy of the city remained, though somewhat reduced: the highest authority was a prince-captain who was always of Venetian birth.
During the Middle Ages and under Venetian rule Split developed into an important port city with trade routes to the interior through the nearby Klis pass. Culture flourished as well, Split being the hometown of Marko Marulic, a classic Croatian author. Marko Marulić wrote Judita (1501) in Split, and published it there (1521). It is widely held to be the first modern work of literature in Croatian. Still, all those achievements were reserved mostly for aristocracy, illiteracy rate was extremely high, mostly because Venetian rule showed little interest in educational and medical facilities.
Venice held Split until its own downfall in 1797. The city fell to Austria-Hungary after a brief period of Napoleonic rule (1806–1813). Large investments were undertaken during that time in the city, new streets were built and parts of the old fortifications were removed.
Under Austria, however, Split can be said to have stagnated. The general upheavals in Europe starting in 1848 gained no ground in Split