The siege of Constantinople was a turning point not only in the history of the Avars, but for the Byzantines, too. The city was first time threatened by siege followed by several attempts of the Muslims. The history of the siege is thoroughly investigated by its written sources, however the identifications of the places mentioned by the contemporain accounts are often dubious. The previous scholarship on the siege neglected a main source of the siege: the Theodosian and Anastasian city walls and in general the defence system of the city, although their analysis can explain itself the outcome of the attack. The aim of my paper is to illuminate a historical event with the light of some topographical and archaeological data, and to offer a better understanding both for the assault and for the function of one of the most significant fortification of the Late Antique world
