The Byzantine castles were the connecting knots of the defensive network of an Empire that was developed in the area of the Eastern Mediterranean, an Empire which neither organized Crusades, in order to solve demographic and financial problems, nor invested its policy in Sacred Wars, but paid the penalty of its non-violent attitude with its destruction and disappearance after eleven centuries of existence. These castles, being stone-built, seem to have been so smoothly incorporated through the years with their natural environment that have been forgotten. Only in 1970 some restoration and research projects have begun. Instructions for building, besieging and defending castles are supplied by numerous Late Antiquity and Byzantine writers, while Procopius in the sixth century A.D. mentions at least six hundred locations in the Balkans, where new fortifications were erected or older ones were improved and reinforced by the Byzantine emperor Justinian. The towns of the Hellenistic and Roman era will be transformed in the course of centuries into walled Medieval settlements or inhabited castles in order to offer protection to the rural population. In the tenth century the Byzantines used the term κάοτρον (= castle) to describe fortified towns. Thrace, being the inland region of the Byzantine capital, also functioned as the protective shield of Constantinople with its many thriving harbours, towns and Medieval castles, such as Pythion and Didymoteichon. The fortification works in Macedonia, which either are added to Hellenistic towns (Thessaloniki, Veroia) or are founded in the years of Justinian (Castoria), are completed in the tenth and eleventh centuries (Serbia) and are further reinforced in the Komnenean period, while some new are built in the time of the Palaeologan dynasty. The travelling artery of the Egnatia road, connecting Constantinople with Dyrrhachion, on the Adriatic coast, passes through Thessaloniki, the second important city of the Byzantine Empire. The fortifications along this route as well as in the inland area are located 35-40 kms. far from each other.