In the early Byzantine period, many of the Classical monuments of Athens were converted into Christian churches. New churches were built, too, mostly in the basilica type. However, it was in the Middle Byzantine period (843-1204) that building activity was at its height/ the most notable features of the Byzantine churches of Athens are the so0called Athenian cupola with its elongated but elegant drum, the plainness of the external surfaces, and the use of brick courses in the masonry. Almost all these building are of the cross-in-square type. The finest examples of the Athenian churches are those of St Nickolas Rangavas in Plaka (1031-1050), Kapnikarea (mid-11trh century), Sts Theodore (Theodoroi) (1065), the main church of Kaisariani Monastery(late 11th century), Our Lady Gorgoepikoos (12th century, now a chapel to Athens Cathedral), and the church of the Holy Apostles, built around 1000 in the ancient Agora in an unusual architectural type with four apses. In the octagonal type of ecclesiastical architecture are the Russian church (Sotira Lykodimou), dating from 1030, and the main church of Dafni Monastery (11th century). Dafni Monastery stands on the Sacred Way, on a site formerly occupied by a temple of Apollo Daphneius. This monument is of the greatest significance because of its mosaics, in which links to the aesthetic values of antiquity can be distinguished. For a collection of portable treasures of Byzantine Athens, we must go to the Byzantine Museum.
On the ground floor are sculptures in stone and wood, icons, and wall-paintings. In three of the rooms are reconstructions of the Early Christian basilica, a cross-in-square church of the main Byzantine period, and a single-aisled post-Byzantine church.
Upstairs are more sculptures and icons, wall-paintings detached from defunct churches, miniature works of art, utensils and ecclesiastical robes.