Slavs had fully settled in the area by the middle of the 6th century and changed the ethnic proportions of the region. With the establishment of Bulgaria in 681 Philipopolis became an important border fortress of the Byzantine Empire. It was captured by Khan Krum in 812 but the region was fully incorporated into the Bulgarian Empire in 834 during the reign of Khan Malamir. It remained in Bulgarian hands for a relatively short time until it was reconquered by the Byzantine Empire in 855-856. Under Byzantine control the city became the centre of Paulician heretics transported from the eastern borders of the empire to serve as military settlers on the European frontier with Bulgaria. From Philippopolis the influence of dualistic doctrines spread to Bulgaria forming the basis of the Bogomil heresy. Under tsar Simeon the Great (893�927) the city and most of the Byzantine possessions in the Balkans were conquered by the Bulgarian Empire. The city remained in Bulgarian hands under Simeon's son, Peter I (927�969).
In 970 the Asian army of the Byzantine Empire under the eunuch Peter was destroyed by the Bulgarians near Plovdiv. The city again came to be known as Philippopolis and became Byzantine in character. Aime de Varennes in 1180 encountered the singing of Byzantine songs in the city that recounted the deeds of Alexander the great and his predecessors, over 1300 years before.
Byzantine rule was succeeded by that of the Latin Empire in 1204, and there were two short interregnum periods as the city was twice occupied by Kaloyan of Bulgaria before his death in 1207. In 1208 Kaloyan's successor Boril was defeated by the Latins in the Battle of Philippopolis. Under Latin rule, Plovdiv was the capital of the Duchy of Philippopolis governed by Renier de Trit, and later on by Gerard de Strem. Bulgarian rule was reestablished during the reign of Ivan Asen II between 1225 and 1229. In 1263 Plovdiv was conquered by the restored Byzantine Empire and remained in
