While the oldest archaeological artifacts found in the area date back to the Neolithic, the name "Millstatt" may refer to the Celtic expression "mils" meaning mountain stream or brook. The Celts entered this region from the 5th century BC onwards, their kingdom Noricum came under control of the Roman Empire in 16 BC. During the Migration Period in the 6th century Slavic tribes settled here in the principality of Carantania, which became a march of Bavaria and the Frankish Empire in the late 8th century. According to legend, a Carantanian duke Domitian († 802?) converted to Christianity and built the first church of Millstatt. He also had one thousand statues of pagan gods ("mille statuae", see the coat of arms) gathered and thrown into the lake. About 1070 the Bavarian Count Palatine Aribo II and his brother Poto established Millstatt Abbey, a Benedictine monastery, in Millstatt including a donation of extensive landed property around the lake and estates in Salzburg and Friuli. Although no document is saved the first monks probably descended from Hirsau Abbey. The monastery church, now parish church of Christ the Savior and All Saints, was erected in the second quarter of the 12th century. It replaced an earlier church from the days of the Carolingian dynasty, of which some cut stone slabs remained in secondary utilization. While the Counts of Gorizia, Ortenburg and Cilli held the office of a Vogt protector the monastic community included up to 150 brothers, who made Millstatt a cultural centre of Upper Carinthia and left a famous codex—the 'Millstatt Manuscript'—in Middle High German language from around 1200. The decline of the monastery in consequence of economic and disciplinary difficulties led to its abolition by Pope Paul II in 1469.
The Habsburg emperor Frederick III, by this time also Carinthian duke and Vogt of Millstatt, had urged on this decision for the sake of his foundation of the knightly order of St. George to which he handed over the
