What makes Diocletian's Palace historically important?
Roman Emperor Diocletian began planning his retirement in the 280s CE — a decade before he actually abdicated — and the palace he built on the Dalmatian coast between 295 and 305 CE was on a scale commensurate with imperial ego. The 30,000-square-metre complex occupied an entire promontory overlooking the sea, combining a military garrison, an imperial residence, two temples, a mausoleum, and the vast underground substructure that levelled the rocky terrain beneath everything else. When Diocletian walked through the Peristyle courtyard to his mausoleum on holy days, his 700-man imperial guard would have lined the colonnades. The Sphinx carved from black Egyptian granite that still crouches beside the cathedral entrance came from Egypt at his personal order.
After Diocletian died around 316 CE, the palace passed through various imperial hands and then fell into disuse. When the Roman city of Salona — 6 kilometres to the north — was sacked by Avars and Slavs around 614 CE, its refugees found ready-made shelter inside the palace walls. They converted the mausoleum into a cathedral, bricked up Roman doorways, and began building houses into the Roman walls and colonnade. This habitation never stopped: the Cathedral of Saint Domnius still functions in the 8th-century octagonal tomb of the emperor who persecuted Christians; restaurants operate beneath Roman arches; residents hang laundry between 4th-century columns.
The physical layers of this 1,700-year habitation are what make the palace extraordinary. The underground cellars (the substructure) reveal the Roman blueprint most clearly — vast vaulted chambers whose room plan exactly mirrors the imperial apartments above, preserved because medieval inhabitants used the basement as a rubbish dump rather than a living space. The chambers were cleared in the 20th century and now offer a walk through intact Roman infrastructure: original stone arches, drainage channels, and storage rooms lit by modern spotlights.
Above ground, the Peristyle courtyard is the emotional and architectural heart of the palace — a colonnaded square where Diocletian would have processed on state occasions, today framed by café tables and the ancient steps of the cathedral's vestibule. The Prothyron arch above the vestibule entrance is one of the finest surviving examples of late Roman architectural ornament. Around the courtyard, medieval and Renaissance additions crowd against Roman columns in a density of accumulated history found nowhere else in Europe.