The Turkic-speaking Bulgars arrived in the territories west of the river Dniester around 670. At the Battle of Ongal they defeated the Eastern Roman (or Byzantine) Emperor Constantine IV in 680 or 681, occupied Dobruja and founded the First Bulgarian Empire. They soon imposed their authority over some of the neighboring tribes. Between 804 and 806, the Bulgarian armies annihilated the Avars and destroyed their state. Krum of Bulgaria took the eastern parts of the former Avar Khaganate and took over rule of the local Slavic tribes. During the Middle Ages the Bulgarian Empire controlled vast areas to the north of the river Danube (with interruptions) from its establishment in 681 to its fragmentation in 1371–1422. Original information for the centuries-old Bulgarian rule there is scarce as the archives of the Bulgarian rulers were destroyed and little is mentioned for this area in Byzantine or Hungarian manuscripts. During the First Bulgarian Empire, the Dridu culture developed in the beginning of the 8th century and flourished until the 11th century. In Bulgaria it is usually referred to as Pliska-Preslav culture.