I can't do better than Wikipedia on the subject of Diocletian's Palace, so here's what it says: After he nearly died of an illness, the Roman Emperor Diocletian (ruled AD 284 to 305), great reformer of the late Roman Empire, decided to retire from politics in AD 305. The Emperor ordered work to begin on a retirement palace near his hometown. Work on the palace began in AD 293 in readiness for his retirement from politics. The palace was built as a massive structure, much like a Roman military fortress. It faces the sea on its south side, with its walls 170 to 200 meters (570 to 700 feet) long, and 15 to 20 meters (50 to 70 feet) high, enclosing an area of 38,000 m² (9½ acres). The palace water supply was substantial, fed by an aqueduct from Jadro Spring. This opulent palace and its surroundings were at times inhabited by a population as large as 8,000 to 10,000 people, who required parks and recreation space; therefore, Diocletian established such outdoor areas at Marjan hill. The palace was finished in AD 305, right on time to receive its owner, who retired exactly according to schedule, becoming the first Roman Emperor to voluntarily remove himself from office. After a few years, a group of Roman Senators came to Diocletian's palace, asking the former emperor to return to Rome and help the Empire to overcome growing political problems. Diocletian refused, and while he was showing them his garden, he told them that he could not leave his beautiful garden which he had created by his own hands.
Over the intervening centuries, the Palace has been claimed by the citizenry, but despite that, much remains of its original fabric: