Founded in 315 before Christ, Thessalonika was for centuries the provincial capital and an important seaport. Due to that the city became one of the first bases for the spread of Christianity. The earliest preserved Christian monuments in Thessalonika were built as early as 4th century. The mosaics of the rotunda (round church) St. Demetrius and St. David church are among the great masterpieces of early Christian art. People continued constructing numerous beautiful churches here also in the following centuries, until Byzantium was conquered by the Turks (in 15th century); by those churches we can trace the development of Byzantine architecture in the Middle Ages. They had considerable influence on arts in the entire Byzantine world.