From the foundation of Constantinople in 330 AD until the end of the reign of Heraclius in 641, the Eastern Roman Empire evolved. It was based on both a strong Hellenistic-Roman ethos (manners and customs) for matters of the state and everyday life, but also on the new outlooks and attitudes brought into being by the spreading of the Christian faith. Amongst the diagnostic and defining features of this period are ambitious and effective emperors such as Constantine the Great, Theodosius the Great, Justinian and Heraclius, catastrophic barbarian invasions and pillaging, natural disasters, internal Christian conflicts, major military campaigns undertaken to restore or preserve territorial integrity and extensive building projects. These last include the erection of majestic Christian churches, and the development and fortification of the towns.

From the late 6th until the middle of the 9th centuries the Byzantine Empire faced challenges by land and by sea. First the Avars and Slavs and then the Arabs sowed disruption and insecurity among the Byzantine populace, not only in the provinces but also in Constantinople itself. In the Peloponnese there occurred demographic, economic and administrative upheavals, as the Slav raiders gradually transformed themselves into permanent residents, adopting the manners and customs of the natives: the Empire regained control of the area at the end of the 8th century. However, the coastlines were then threatened by the Arabs, especially after the second decade of the 9th century, when Crete passed into their hands and acted as a base for their predatory actions. Simultaneously, a theological controversy centred upon the validity and correctness of worshipping God through the medium of images exploded into widespread civil conflict, the iconoclasm: this tore the empire asunder between 726 and 843.

The Eastern Roman Empire managed both to overcome the internal civil strife engendered by Iconoclasm and to prevail against its external enemies. It controlled the Slavic elements within and without its borders, repelled the Arabs from Asia Minor, reoccupied Crete and Palestine. The coasts were now free from pirates; security and commercial activity strengthened. By now the late Roman Empire has mutated into the Byzantine: it has lost the Latin language and polytheism, in their place prevailed Hellenism and Christianity. The ecclesiastical and secular architecture, with the visual arts and minor crafts, flourished. Regional Byzantine cities differed from their ancient counterparts by being much smaller and walled; they were organized not around the market (Agora) but were centred upon the Christian church. The empire reached its peak around the end of the 10th century to the mid 11th. But from the late 11th century, Byzantium lost both territories as the Turks settled in Asia Minor and economic power as Italian trading city-states acquired more and more privileges at the expense of the Byzantine merchant-class. The middle farming class became impoverished, to the advantage of the local landowning aristocracy, the Nobles.

Byzantium was devastatingly affected by the Fourth Crusade. The Crusaders occupied Constantinople in 1204: they proceeded to share out the territories of the empire amongst themselves. This was especially so in the Balkans with Partitio Terrarum Imperii Romaniae. Constantinople herself became the seat of the Latin emperor, Thessaloniki acquired a Latin king with jurisdiction in the territories of the southern Balkans and with vassals holding the Barony of Salona, ​​the Marquisate of Boudonitza and the Duchy of Athens, whilst in the Peloponnese was set up the Frankish principality of the Morea, where the mingling of western and local peoples created a mixed Franco-Byzantine society. The Venetians held key positions on the coast of the mainland and the islands to control trade. Three major Greek states were created: the Empires of Trebizond and of Nicaea in Asia Minor and the Despotate of Epirus. Among regional rulers who held power for a short time was Leon Sgouros in the area of Corinth, Argos, Nafplion, etc. In the Peloponnese in 1262 was created the Despotate of Mystras, which gradually extended its sway across most of the Peloponnese. The Latins and Byzantines both established and repaired numerous fortifications, of use in their countless martial conflicts that only came to an end in 1460 with the final victory of the Turks in the Morea.

Since 1071, after the Battle of Manzikert, the Seljuk Turks had gradually consolidated their presence in Asia Minor. From the late 13th century the Ottoman Turks spread rapidly across the lands of the fragmented Byzantine Empire, prevailing over the Byzantine or Latin states of the region, which were busy fighting each other. Constantinople became their capital in 1453, while the Peloponnese had passed almost entirely into their hands by 1460. The only people who were able to offer effective resistance to the Turkish expansion were the Venetians, but eventually they too lost control of Cyprus and Crete. Though the Peloponnese was recovered in the late 17th century, it was lost again and permanently in 1715. The Venetians and Turks fortified every strategic position that fell into their respective territories.

In 1821 yet another revolution against the Ottomans broke out. After countless battles against the Ottomans, but also against disaffected Greeks, there was created the new Greek State in 1830. It embraced the Peloponnese, Central Greece and the Cyclades. Its economic situation was wretched, but it managed slowly but steadily to expand until it had acquired its current territorial extent. The last time the old fortifications were used for military purposes was during the Revolution. In the 19th century they served as prisons or barracks. The old castles were at times demolished deliberately and at others just abandoned and left to their fate, re-emerging today as but the shadows of bygone greatness.