Leros is one of the islands in the Dodecanese, at the eastern edge of the Aegean. Archaeological evidence has revealed the active presence of humans there since prehistoric times, in finds dating to the late Neolithic and Early Bronze Age. In historical times the evolution and character of Leros were heavily influenced by its close relations with Miletus, whose people settled the island. It later came under Spartan and then Roman rule, while archaeology has also confirmed the presence of Macedonians. The official religious shrine in the classical era was at Partheni, in the northern part of the island. This was associated with the worship of a maiden called Iocallis, who connects the island with the myth of the Meleagrids and is considered synonymous with the goddess Artemis. Architectural remains from antiquity include two major fortifications at Partheni and Xirokambos, as well as the island’s principal settlement at Agia Marina.

The transition to the Christian era found Leros being administered as part of the Province of the Islands (Provincia Insularum), under the command of Asia Minor. The spread of Christianity throughout the Dodecanese did not exempt Leros, if at some delay, since the cult of Artemis remained strong for a considerable time. After the new religion prevailed the island was organized into a diocese. The Diocese of Leros formed part of the Metropolis of Rhodes, and was definitely founded before 553, when Bishop John I of Leros took part in the Fifth Ecumenical Council of Constantinople convened by Justinian to condemn heretical writings.

The early Christian period was marked by intense building activity. Factors such as the dominance of Christianity and the organization of living areas favoured the construction of numerous basilicas and the establishment of many new settlements, at Agia Marina, Partheni, Alinda and Lakki. The archaeological record from this period is not merely limited to abundant architectural remains, which testify to extensive rebuilding on the island, but also includes artefacts (coins, pottery), numerous mosaic fragments, votive inscriptions and scattered architectural members.

The era of prosperity experienced by the islands in the first Christian centuries was violently interrupted by the invasion and advance of Arabs in the Aegean. The years that followed up until the 10th century were ones of decline and stagnation. Sea routes became dangerous, commercial activity grounded to a halt and many settlements and trading posts were laid waste. On Leros this turbulent period was reflected in the abandonment of many villages, as the population moved into the island’s interior. Both Lepida Castle and Panteli Castle, the most important medieval monument on Leros, have construction phases dating to this period.

This dark era came to an end when Emperor Nicephorus Phocas recaptured Crete in 961, ushering in a period of renewed Byzantine power in the Aegean Sea. Large monasteries were erected on the islands and communications were re-established with the major centres in the empire. Imperial documents from the 11th century testify to developments on Leros. Typical of these was the chrysobull issued by Emperor Alexius Comnenus in 1087, granting the island of Leipsoi, the suburbs of Partheni and Temenion and part of Panteli Castle on Leros to Christodoulos Latrenus, founder of the Monastery of Agios Ioannis Theologos on Patmos – the following year he was granted the whole castle, on the order of Anne Dalassene. In 1309 the island of Leros was occupied by the Knights Hospitaller and incorporated into the region of Kos, together with the neighbouring islands of Kalymnos and Nisyros. From the late 15^th to the early 16^th century the knights carried out repairs to the walls and built an external wall around Panteli Castle, in order to update the island’s defences following the introduction of gunpowder weapons. Systematic pirate raids and looting in the second half of the 15th century led to Leros falling to the Ottomans in 1522.