In the region between Mt. Oeti and Mt. Parnassus, in the land of the Dryopes according to tradition, the Dorians founded four cities, namely Kytinion, Voion, Erineos and Akyphas-Pindos, which constituted the Doric Tetrapolis.  Doris was one of the oldest members of the Delphic Amphictyony and, according to Thucydides, it was an important and strategic region already 25 years before the Peloponnesian War, the first time when the Phoceaens and the Lacaedemonians first clashed against each other, the former as invaders and the latter as protectors of the Doric capital Kytinion. In the 3d century B.C. the Doric Tetrapolis joined the Aetolian League. 
In the 6th century A.D. the ancient Voion was the only one of the cities of the Doric Tetrapolis mentioned by Hierocles in his "Synecdemus" (535 A.D.). The region of Doris took the Slavic name “Gravia” and in 1254 the king of Thessaloniki Boniface of Montferrat founded the Frankish barony of Gravia. 
Text - Translation: Dr. Aphrodite Kamara, Historian