Dinaric Alps and most of the eastern Adriatic coast. The historian Theodor Mommsen wrote in his book, The Provinces of the Roman Empire, that all Dalmatia was fully romanized by the 4th century AD. However, analysis of archaeological material from that period has shown that the process of Romanization was rather selective. While urban centers, both coastal and inland, were almost completely romanized, the situation in the countryside was completely different. Despite the Illyrians being subject to a strong process of acculturation, they continued to speak their native language, worship their own gods and traditions, and follow their own social-political tribal organization which was adapted to Roman administration and political structure only in some necessities. The fall of the Western Roman Empire, with the beginning of the Migration Period, left the region subject to Gothic rulers Odoacer and Theodoric the Great. They ruled Dalmatia from 480 to 535 AD, when it was restored to the Eastern Roman (Byzantine) Empire by Justinian I. Middle Ages The Middle Ages in Dalmatia were a period of intense rivalry among neighboring powers: the waning Byzantine Empire, the Kingdom of Croatia (later in a personal union with Hungary), the Kingdom of Bosnia, and the Republic of Venice. Dalmatia at the time consisted of the coastal cities functioning much like city-states, with extensive autonomy, but in mutual conflict and without control of the rural Hinterland. Ethnically, Dalmatia started out as a Roman region, with a Romance culture that began to develop independently, forming the now-extinct Dalmatian language. In the Early Medieval period, Byzantine Dalmatia was ravaged by an Avar invasion that destroyed its capital, Salona, in 639 AD, an event that allowed for the settlement of the nearby Diocletian's Palace in Spalatum (Split) by Salonitans, greatly increasing the importance of the city. The Avars were followed by the great South Slavic migrations. According to the work De Administrando Imperio written by the 10th-century Byzantine Emperor Constantine VII, the Croats had arrived in Roman province of Dalmatia in the first half of the 7th century. and the Serbs settled in Dalmatia south of the Cetina river, in Pagania, Zahumlje, Travunia and Konavle. The Slavs, loosely allied with the Avars, permanently settled the region in the first half of the 7th century AD and remained its predominant ethnic group ever since. The Croats soon formed their own realm: the Principality of Dalmatian Croatia ruled by native Princes of