Roman Dalmatia was fully Latinized by 476 AD when the Western Roman Empire disappeared During the Barbarian Invasions, Avars allied with certain Slavic tribes, invaded and plundered Byzantine Illyria. This eventually led to the settlement of different Slavic tribes in the Balkans. The original Roman population endured within the coastal cities and in the inhospitable Dinaric Alps. The Dalmatian cities retained their Romanic culture and language in cities such as Zadar, Split and Dubrovnik. Their own Vulgar Latin, developed into the Dalmatian language, a now extinct Romance language. These coastal cities (politically part of the Byzantine Empire) maintained political, cultural and economic links with Italy, through the Adriatic Sea. On the other side communications with the mainland were difficult because of the Dinaric Alps. Due to the sharp orography of Dalmatia, even communications between the different Dalmatian cities, occurred mainly through the sea. This helped Dalmatian cities to develop a unique Romance culture, despite the mostly Slavicized mainland.