Most Roman monuments still standing have reached us because someone decided to preserve them. This one arrived for the opposite reason: because nobody ever left it alone.
For seventeen hundred years people have lived inside it, and that has done more for its survival than any protection plan.
01 Two hundred and fifteen metres by a hundred and seventy-five
It is worth starting with the scale, because it explains the rest. The plan is an irregular rectangle: 214.97 metres on the east side, 174.74 on the north and 181.65 on the south, adjusted to the ground.
Around it stood sixteen towers projecting from the western, northern and eastern façades, plus four corner towers. That combination — walled rectangular enclosure, corner towers, gates on the axes — is not a palace's: it is a Roman military camp's.
And half of it was. Roughly half the complex was for the emperor's personal use and the rest housed the garrison. The building was constructed as a retirement residence and a fortress at once.
Seventeen hundred years of people living inside have preserved the building better than any protection plan.
02 The move that changed everything
Six kilometres from here stood Salona, the capital of the province of Dalmatia and one of the largest cities of the late empire, with some 60,000 inhabitants. It was the reference city for the whole coast.
In the seventh century, Salona was destroyed. And the people who left it moved into the most solid thing at hand: a walled enclosure, empty, with towers, gates and walls several metres thick.
From then on the building stopped being a building. Halls were divided into dwellings, courtyards into squares, porticoes into streets. Each generation added floors, blocked arches, opened windows and leaned a new house against a thousand-year-old wall.
03 The streets were not laid out: they were already there
What makes this place unique is not that there are Roman ruins in the old town: it is that the old town has the shape of the Roman building. The two main streets crossing it are the enclosure's original axes, running gate to gate.
The crossing of those two axes is the peristyle, the colonnaded courtyard that was the emperor's ceremonial antechamber. Today it is a public square with steps where people sit, ringed by granite columns brought from Egypt.
And the blocks of houses are the palace's former compartments. That is why the streets inside are straight and meet at right angles, while outside the wall the old town is a tangle of medieval alleys. The difference is obvious the moment you cross a gate.
04 The cellars, which keep the plan of the floor above
The most useful part of the complex, for understanding what was above, is below. The cellars are a system of barrel-vaulted rooms built to level the ground and support the imperial apartments.
For centuries they were used as a dump and became choked with debris. That neglect was a stroke of luck: while everything above was being rebuilt, below the plan survived intact under the rubble.
Because the cellars reproduce the layout of the rooms they supported, today they serve as a plan of the original palace. It is one of the few parts of the building where you can walk through a space that keeps its Roman size and shape.
05 The stone came from the island opposite
The materials all came from very close by, and that explains how a work of this scale could be raised in a decade. The white limestone comes from Brač and in part from Seget, next to Trogir.
Tufa was taken from nearby riverbeds and brick was made in Spalatum and in workshops round about. Only specific pieces, such as the granite columns and the sphinxes, travelled from far away.
And something similar happened with the workforce. The Greek names carved in the stone, Zotikos and Filotas, indicate that Diocletian brought masters from the east of the empire, but a good part of the workers were local. The building is, essentially, the island opposite cut into blocks.
06 What to look for on the way in
First, the joints. On almost any wall inside the enclosure you can see three or four periods stacked on the same course: Roman ashlar at the bottom, medieval brick in the middle, modern render above and an electric cable crossing the lot.
Second, the blocked arches. The palace's inner façades had large arcades in their upper part, and many of them were closed up when they became house walls. The arch is still there, drawn on the wall, with a square window in the middle.
And third, the difference in layout. Go out through a gate and walk a hundred metres: the streets stop being straight all at once. That invisible line is the wall, and it separates the city built inside a building from the city built without one.
