Thessalonike played an important role in historic events that occurred during the Byzantine period. Its strongly fortified walls resisted a succession of barbarian hordes, and were overrun only four times in the course of a thousand years.
As an administrative centre Thessalonike was the hub of many state services and military commands, while the products of the Balkan Peninsula passed through her port. A metropolitan see with twelve bishoprics dependent upon it, it was honoured by the presence of men of letters (e.g. Ioannis in the 7th century, Leon the Mathematician in the 9th, Eustathios in the 12th, Grigorios Palamas in the 14th, and Symeon in the 15th century).
Thessalonike also played a leading part in the conversion of the Slavs to Christianity: the brothers Cyril and Methodios, missionaries to the Slavic peoples in the 9th century, were born and bred in Thessalonike.
From the Middle Byzantine period onwards it was the scene of intense cultural activity, comparable with that of Constantinople, its influence being felt even on Mount Athos. Despite religious conflicts (Hesychasm), civil wars and social disorders (the Zealots), Thessalonike was at its peak in the 14th century, as is evident most particularly in the architecture, wall-paintings, and mosaics.
Following disturbances at the end of the 14th century and a brief occupation by the Venetians (1423-30), the city was annexed by Murad II in 1430 and entered upon a new phase of its history.