Of course Athens is well known for its illustrious ancient past, and as the modern capital and largest metropolis of Greece. However, despite being less noteworthy it still existed during the medieval period. This slideshow I made using pictures from ancientathens3d gives a glimpse into the medieval Acropolis of Athens. The music is supposed to be actual ancient music by Michael Levy, the Epitaph of Seikilos.
The rise of Christianity was particularly hard on Athens, as it had such a famous pagan history and had been a center of pagan learning. Justinian closed the famous academy in 529. The Parthenon was turned from temple to church. The greatest artworks were destroyed or brought to Constantinople, possibly including the famous Athena Parthenos (but if so crusaders destroyed it). Athens lost its importance, being purely a regional town. It still had noble aristocratic families and produced two empresses in Irene of Athens and Theophano of Athens (short lived). For much of the Byzantine period it did not prosper but after the loss of Asia Minor to the Turks and the instability that caused, Athens and Greece flourished economically by escaping that chaos. From the 11th to 12th centuries Athens grew rapidly. But then came the Frankokratia.
The Fourth Crusade created several Crusader states in Greece, and Athens was never liberated by the Byzantines except briefly under Constantine XI before he was emperor. But the Duchy of Athens was an Ottoman vassal and the Turks sent a massive army to prevent any Byzantine revivals. In 1458 the Ottomans seized the city, ejecting the crusaders once and for all. The Parthenon was converted from a Catholic Church to a Ottoman Mosque. The city remained in Turkish hands until it became the capital of the Kingdom of Greece following the Greek Revolution.