ancient Roman palace built between AD 295 and 305 at Split (Spalato), Croatia, by the emperor Diocletian as his place of retirement (he renounced the imperial crown in 305 and then lived at Split until his death in 316). It was both an imperial city-palace and a sea fortress, as well as a country house of vast proportions and magnificence, covering an area of 9 1/2 acres (four hectares). The north to south wall measured 705 feet (215 metres), with the walls being 7 ft thick and 72 ft high on the Adriatic side and 60 ft on the north. There were 16 towers and four gates: Porta Aurea (north), Porta Argentea (east), Porta Ferrea (west), and Porta Aenea (south). The rectangular ground plan was like a Roman camp; i.e., with four arcaded avenues 36 ft wide meeting in the middle. The imperial apartments were in the two southern quadrants, along the width of which ran a 524-ft-long and 24-ft-wide arcaded grand gallery (probably for promenades and display of art) that was open to scenic views of the sea and the Dalmatian coast. The Temple of Jupiter and Diocletian's mausoleum (a cathedral after the 7th century) were located in courts of the imperial section. Guests and household officials were accommodated in the northern quadrants.

The Avars badly damaged the palace, but when their incursion was over (639) the inhabitants of the ruined city of Salona (Salonae; Diocletian's birthplace) took refuge within what remained of the palace and built their homes, incorporating the old walls, columns, and ornamentation in their new structures (this area is now the "Old Town" of Split).

window with a somewhat rounded top, or head, and bronze-framed panes of glass, named after those in the palace of the 3rd-century Roman emperor Diocletian at Spalato (Split, Croatia) and in the Baths of Diocletian, Rome (now the church of Sta. Maria degli Angeli). The window was used again in the 16th century, especially by Andrea Palladio, and in the late 18th century by Robert Adam.