The ruins of Diocletian's Palace, built between the late 3rd and the early 4th centuries AD, can be found throughout the city. The Roman Emperor Diocletian spent his declining years in an enormous palace that he had built near his birthplace, Aspalthos, in Dalmatia. The palace represents the most valuable example of Roman architecture on the eastern coast of the Adriatic. Its form and the arrangement of the buildings within the palace represent a transitional style of imperial villa, Hellenistic town and Roman camp.
On the eastern side of the palace lies the Porta Argentea (Silver Gate) with the church of St Dominic on the opposite side, it was reconstructed between 1932 and 1934. The Silver Gate gives access to the Plain of King Tomislav and thence to the Peristil (peristyle), the central open-air area of the palace. Its longitudinal sides are surrounded by an arched colonnade; the arches in the west are closed by Gothic and Renaissance houses. monumental port with four columns carrying a gable closes the Peristyle in the south.
The Mausoleum of Diocletian (today's Cathedral of St Doimus dedicated to St Mary) lies in the eastern part of the peristyle. The mausoleum has almost completely preserved its original octagonal form, encircled by 24 columns which supported the roof; the interior is round, with two rows of Corinthian columns and a frieze. A dome, once covered with mosaics, roofs the mausoleum. The monumental wooden gateposts and the stone pulpit from the 13th century represent the oldest monuments in the cathedral. The choir, constructed in the 18th century, is furnished with Romanesque seating from the 13th century and ornamented with a painting representing the Mother of God with the saints and donors.
A small temple opposite the mausoleum, probably dedicated to Jupiter, became the baptistry in the early Middle Ages. Only the closed part of the temple (cella) with a richly decorated portal has been preserved; the interior is roofed with a barrel-coffered vault.
Diocletian Street runs from the Peristyle to the north at the Porta Aurea (Golden Gate); Agubio Palace, with a Gothic portal and inner yard is to the left. To the right, in Papaliceva Street, is the Papalic Palace (15th century), the most important example of Gothic architecture in Split.