From the Byzantine Empire to the Ottoman conquest
With the partition of the Roman Empire and the rise of Byzantium in the 4th century AD, the Graeco-Roman world underwent major economic, social, cultural and religious changes. Whereas northern Greece experienced numerous waves of invasions by Slavic and Germanic peoples, Euboea seems to have been spared. Because of its strategic position between the Aegean Sea and continental Greece, the island constitutes a genuine intersection. The fertility of its plains (especially the Lelantine plain), its timber resources in the north (important for naval construction), as well as the abundance of shellfish (indispensable for the dye works of Chalkis and Thebes) all gave it a significant economic role. While earlier Euboean centers shifted or disappeared, like Eretria in the 6th century, others endured, such as Chalkis, where the Emperor Justinian (527-565 AD) restored the fortifications and the bridge over the Euripos in order to control the sea lane of the Euboean Gulf.
After the reorganization of the Byzantine Empire, Euboea became part -under the reign of Justinian II (685-695), at any rate- of the province of Hellas, whose capital was Thebes. Chalkis then served as a harbor for its fleet.
On the ecclesiastical level, Euboea had five dioceses: Euripos (Chalkis), Oreos, Karystos, Porthmos (the port of modern-day Aliverion) and Aulon (today Avlonari). The numerous monasteries and churches built between the 4th and the 12th centuries, such as Aghios Georgios Arma and Aghios Nikolaos above Amarynthos, testify to the wealth of Byzantine culture. During the period of Latin domination, Euboea was a major papal fief, notably from the 14th century to the Ottoman conquest, since the Latin patriarch of Constantinople resided in Chalkis.
Venice and the Ottomans
From the 11th century, the Byzantine Empire had to repel Norman incursions. Only the help of the Republic of Venice could preserve Byzantium's sovereignty. In exchange for its support, the Serenissima demanded favorable commercial agreements and succeeded in strengthening its position in the Aegean before the conquest of Constantinople in 1204. In 1366, Venice gained control of Euboea, previously administered by the Franks established in Oreos, Chalkis and Karystos. However, Venice did not aspire to control the whole of the territory and was content to hold a few strategic points, where impressive fortifications still remain today, such as La Cuppa, near Kymi, and Phylla, in the Lelantine plain.
On July 12, 1470, Sultan Mehmed II (1432-1481) seized the city of Chalkis and incorporated Euboea into his empire. Like Venice, the Ottomans chose Chalkis as their administrative center and controlled strategic points in northern, central and southern Euboea. They ruled the island until 1832.