In 395 AD, the Roman Empire was divided into two parts - the West Roman and the East Roman (Byzantine) Empire. This division was made after the death of Emperor Theodosius I, the last emperor to rule over the united empire. Theodosius divided the empire between his two sons: Honorius received the western part with its seat in Rome, while Arcadius became ruler of the eastern part, whose capital was Constantinople.

For the island of Kos and other areas of the eastern Mediterranean, this division marked the beginning of the Byzantine period. Kos became part of the Eastern Roman Empire, which continued in many ways the heritage of ancient Rome, but also gradually became more oriented towards the Greek language and Christian culture. Christianity began to develop on Kos as early as the late Roman Empire, but during the Byzantine period it became the dominant religion and greatly influenced the island's daily life, culture and architecture.

Early Byzantine period (4th - 7th century)

At the beginning of the Byzantine period, Kos retained its importance as a strategic point in the Aegean. The Byzantine Empire, which became a Christian power, encouraged the construction of religious buildings on the island. An important monument from this period is the Basilica of Agios Stefanos (St. Stephen), built in the 5th to 6th centuries, which is one of the oldest Christian buildings on the island and attests to the early adoption of Christianity on Kos.

The island continued to prosper as an agricultural and commercial centre. The port of Kos served as a stopover for ships sailing between Asia Minor and mainland Greece. The city of Kos retains some of its Roman and Hellenistic character, although many of the Roman buildings have been converted or adapted for Christian use.

In 554, a very strong earthquake struck the island of Kos, which also affected other areas in the Aegean and on the coast of Asia Minor. The earthquake had a devastating impact on the architecture and infrastructure of Kos. Many buildings, including religious buildings, public buildings and the city walls, were severely damaged or completely destroyed. The island's recovery from this natural disaster took a long time, which influenced its further development during the Byzantine period. This event highlighted the island's vulnerability to earthquakes, which are common in this geologically active area.

Middle Byzantine period (7th - 12th century)

During this period, Kos, as well as the whole Aegean Sea, faced invasions by Arab invaders who launched attacks on Byzantine territories. The island was attacked several times and in the 7th century had to adapt to a new reality that required improvements in its defensive structures.

New fortresses were built on Kos, such as the fortified town of Paleo Pili, which served as a refuge for the inhabitants during the raids. Many of these fortresses were located inland, in strategic locations where it was easier to defend. As a result of these changes, some of the population moved from the coastal areas to safer locations inland.

In the Middle Ages, urban life declined and Kos became a more isolated and rural island, although important urban centres continued to exist. The culture of the island was increasingly influenced by Christianity, which was reflected in the architecture, with older pagan temples being converted into churches.

Christianity gradually became central to the social and cultural life of the people of Kos during the Byzantine period, and Christian values began to form an integral part of their daily lives.

Late Byzantine period (12th - 14th century)

In the late Byzantine period, especially after the Fourth Crusade in 1204, when Constantinople was temporarily controlled by Latin crusaders, the Byzantine Empire weakened considerably and lost control over many of its territories. The island of Kos became the subject of disputes between the various powers at this time.

The Byzantine period ended in 1315, when Kos came under the control of the Knights of the Order of St. John, who controlled it until the 16th century. This change brought new influence, with the island serving as a military base against the Ottoman Empire. The Knights built several new fortresses on the island, the most important of which is Neratzia Castle, which guards the harbour in the capital of Kos. However, Byzantine influence in culture, religion and daily life persisted throughout the Knights' period.

Sights

Basilica of Agios Stefanos - This early Christian basilica dates from the 5th to 6th century AD and is located on the Kastri peninsula near Agios Stefanos beach. It is one of the oldest Christian buildings on the island. Although it is now a ruin, some parts of the mosaics and foundations have been preserved, showing the original size and importance of this church.

Paleo Pili - Paleo Pili is a fortified Byzantine town located on a hill in the central part of the island. It was built in the 11th century as a refuge for the inhabitants during raids and attacks. Remains of walls, churches and other buildings have been preserved.

Church of Agios Ioannis - The monastery and church of Agios Ioannis (Saint John) is located on the slopes of Mount Dikeos. Although it dates from a later phase of the Byzantine period (approximately 11th to 13th centuries), it is an important example of religious architecture of the period. The church is known for its simple but beautiful architecture and has spectacular views of the surrounding countryside.

Remains of the city walls - In the capital of Kos, parts of the Byzantine walls that protected the city from enemy raids have been preserved. These walls were strengthened and extended in some places during the subsequent rule of the Knights of St. John.