Diocletian's Palace was conceived at the end of the third century as a unique combination of imperial residence, ceremonial court, fortified compound, and military installation. Emperor Diocletian, born in the Roman province of Dalmatia, ordered it built near his native region and close to Salona, the provincial capital. Work probably began around 295. The immense rectangular complex faced the Adriatic, with monumental gates on the landward sides and a southern façade opening toward the sea. One half contained the emperor's apartments and ceremonial spaces; the other accommodated guards, servants, storage, and the machinery of an imperial household.

On 1 May 305 Diocletian voluntarily abdicated, an extraordinary act for a Roman emperor, and retired to the palace. He had reorganized the empire through the Tetrarchy and had also authorized the last major imperial persecution of Christians. At Split he spent his final years amid gardens and architectural imagery that presented him as a divinely favored ruler. His mausoleum stood beside the Peristyle, while temples, including one dedicated to Jupiter, formed part of the sacred plan. Diocletian died early in the fourth century, and the palace remained imperial property, used at times by officials, exiles, and members of the ruling establishment.

The complex underwent its decisive transformation after Salona was attacked and abandoned during the upheavals of the seventh century. Refugees moved into the defensible ruins, adapting corridors, substructures, guardrooms, and apartments as houses, workshops, streets, and churches. The emperor's mausoleum became the Cathedral of Saint Domnius, honoring a Christian martyr associated with Diocletian's persecution. The Temple of Jupiter became a baptistery. This conversion reversed the religious meaning of the palace and preserved major Roman structures by giving them essential roles in the medieval city.

Over succeeding centuries Split grew within and beyond the Roman walls. Romanesque churches appeared in former passages and gate complexes. Noble families built Gothic, Renaissance, and Baroque residences against ancient masonry. The Peristyle remained the civic and ceremonial heart of the settlement. Byzantine, Croatian, Venetian, Hungarian, and Habsburg authority each shaped the city, but none erased the palace's underlying plan. Medieval fortifications extended its defenses, while commerce through the harbor sustained an urban community rather than an abandoned archaeological monument.

Scholars and architects increasingly studied the palace from the Renaissance onward because its surviving vaults, façades, and plan offered unusually direct evidence of late Roman design. Conservation and excavation exposed the extensive substructures beneath the former imperial apartments, whose layout helped reconstruct the vanished rooms above. In 1979 UNESCO inscribed the Historical Complex of Split with the Palace of Diocletian on the World Heritage List. The site's importance lies in continuous adaptation: a monument created for one emperor became shelter for refugees, then the framework of a medieval and modern city. Roman imperial ambition, Christian memory, and ordinary urban life remain physically interwoven in its walls. Everyday adaptation remains visible in the fabric: ancient columns support later houses, narrow lanes follow Roman corridors, and shops occupy vaulted substructures once used to support the imperial apartments. This living continuity distinguishes Split from a palace preserved only as a ruin and explains why conservation must serve residents as well as visitors.