The city of Split was developed from the palace to which Emperor Diocletian retired after abdication in 1305. Split acquired the attributes of city after the fall of nearby Salona about AD 640. In the 7th century, large groups of Avers and Slavs infiltrated the area of Kaštela bay who devastated the city of Salona extensively. The surviving population fled to the nearby islands and significant number found shelter within the walls of the palace. The imperial palace, owing to its powerful walls and defensive towers, was quickly transformed into the Mediaval city of Split.
The Roman buildings were partitioned into smaller wholes and become the basis for a new urban network. Diocletian,s mausoleum was turned into a cathedral consecrated to the holy Virgin, while from the 9th century there is more and more mention of the church of St. Duiam, the first of Salona bishop and Christian martyr who died during the time of Diocletian,s bitter persecution of Christians. During the Middle Ages Split was for the most time under Byizantine rule. In the 9th century, the kingdom of Croatian dukes came into being growing particularly strong in the 10th century. In the 11th century Split was a part of Croatian state under king Petar Krešimir IV. As early as 1108, when King Koloman confirmed the city privileges of Split, during the reign of the Croatian – Hungarian rulers, Split had status of a free commune which brought about prosperity.
From 1420 Split was under the rule of the Venetian Republic and remained within its state framework until its fall and demise in 1797. The first onslaughts of the Turks were recorded around 1500. From that time, for full two centuries, Split and its vicinity were in immediate danger of being conquered. Nevertheless, in times of greater peace it developed trading ties with the interior. During the period of imminent Turkish danger, the Venetian administration in the 17th century finished a powerful system of fortresses within pentagonal bastions encompassing the whole city, while self-standing fortifications were built on the high ground Gripe and on the southeastern cape of Split harbor. After Napoleon’s annihilation of the Venetian Republic in 1797 Split was for a short period of time under Austrian monarchical rule, while from 1806 to 1813 it was under Napoleon’s administrative power. As the whole of Dalmatia, from 1813 to 1918, Split was under Austrian rule. The face of the city changed in the 19th century. It expended far outside the perimeters of the old Medieval core and a modern harbor with a breakwater was constructed.
According to international conventions, in 1979 UNESCO added the historical core of Split with Diocletian’s palace into the register of World Heritage Sites.
The complex of Diocletian has the form of a somewhat deformed rectangle. The longer sides are 215 m long while the northern and southern ones are approximately 180 m in length. The total surface of the palace complex is around 30,000 square meters. It was built according to the taste and the ideas of the eastern part of the Roman Empire and at the same time represents a very luxurious imperial villa, a Hellenic city and a strongly fortified army camp. The powerful outer defensive wall was fortified at the corners with four massive square towers. The palace was protected by altogether 16 towers. The palace is today the best preserved complex of Late Roman imperial architecture in the world.