 The Carolingian Empire arose in the last part of the period and subsequently the Frankish Kingdom of Italy took control of the Adriatic Sea's western coast extending to the Kvarner Gulf, while Byzantine control of the opposite coast gradually shrunk following the Avar and Croatian invasions starting in the 7th century.The region was gradually incorporated into the medieval Kingdom of Croatia by the 11th century, when the kingdom reached its territorial peak, and the city of Senj became the most important centre of the region.Items of significance to Croatian historical heritage originated from the region in that period.