text: Bulgaria: The Bulgars settled the Balkans permanently after 680. They invaded from northeast from the territories of Old Great Bulgaria and created the First Bulgarian Empire uniting with the numerous local Slavs. Bulgaria became officially Christian in the late 9th century. During the 9th and 10th century, Bulgaria at the height of its power spread from the Danube Bend to the Black Sea and from the Dnieper River to the Adriatic Sea and became an important power in the region competing with the Byzantine Empire. It became the foremost cultural and spiritual centre of Slavic Europe throughout most of the Middle Ages thanks to the Golden Age of Bulgaria. The Cyrillic was developed in the Preslav Literary School in Bulgaria in the late 9th - beginning of the 10th century. The Bulgarian Church was recognized as autocephalous during the reign of Boris I of Bulgaria and became Patriarchate during tsar Simeon the Great, who greatly expanded the state over Byzantine territory. In 1018, Bulgaria became an autonomous theme in the Roman empire until the restoration by the Asen dynasty in 1185. In the 13th century Bulgaria was once again one of the powerful states in the region. By 1422 all Bulgarian lands south of the Danube became part of the Ottoman state, however local control remained in Bulgarian hands in many places. North of the Danube, Bulgarian Boyars continued to rule for the next three centuries. Bulgarian language continued to be used as the official language north of the Danube until the 19th century.