According to Herodotus (VIII, 46) the oldest inhabitants of the island were Dryopians. Stephanus of Byzantium relates that the leader of the Dryopians who emigrated to Kythnos towards the end of the Late Bronze Age was called Kythnos, from whom the island took its name. In the beginning of the historical period new immigrants populated the island. This time they came from Athens (Dio Chrys., Ethnika, 80, 26, 9) under the leadership of Kestor and Kephalenos (Eust., 525). Herodotus (VIII, 46, 48, 66) relates that in 480 B.C. the Kythnians participated in the naval battle of Salamis with two ships. After the Persian wars Kythnos was incorporated in the First Athenian League. It seems that the Kythnians maintained close relations with the Athenians until at least the rise of Macedonian power at which time it seems that they took the side of King Philip B' of Macedon. In 314 B.C. the Cycladic islands formed a confederation which was under the control of Antigonos the One-Eyed (the so-called Nesiotic League which was based on the island of Delos). An inscription confirms the participation of the Kythnians to the League (D.J. Geagan, “Inscriptions from Nemea”, Hesperia 37 (1968) 381-384. Kythnos doubtless remained under the rule of the Macedonians until 288 B.C. when most Cycladic islands became part of the kingdom of the Ptolemaioi of Egypt (IG XII 7, 506; Syll.3, 390 (IG XII, 5, testimonia 1312). According to Titus Livius (XXXI, 15), in the end of the 3rd c. B.C., between 205-203/202 King Philip E' of Macedon achieved in annexing the Cyclades to his kingdom and established in 201 B.C. garrisons on Andros, Paros and Kythnos. Shortly afterwards the Rhodians, with their ally King Attalos A' of Pergamon attempted to seize the islands from the Macedonians. They coalesced with the Romans and after seizing control of Andros they attacked Kythnos; they besieged the capital of the island in 199 B.C. but after a few days they left without achieving to capture the city (Tit. Liv. XXXI, 45). However, Kythnos seems to have fallen under Rhodian influence a few years later, since in an honorific inscription of the early 2nd c. B.C. found on Lindos in Rhodes, a Rhodian General, Agemachos, is honoured by the Kythnian demos (IG XII, 5, testimonia 1347). During the Roman period Kythnos, like the other Cycladic islands, declined and served either as a place of exile or as a base for pirates.
The oldest evidence for occupation of the island dates to the Mesolithic period, at the site of Maroulas, where burials and habitation structures were excavated. Kythnos was important in the Bronze Age, especially in the third millenium B.C., due to the exploitation of its copper resources (site de Skouries). An acropolis of the Late Bronze Age existed at the Kastro tis Orias, in the place where the Medieval capital was later established. An Archaic fortified settlement has been identified at Kastellas. All over the island more than 30 towers have been identified.
The ancient capital of Kythnos (modern Vryokastro) is situated on the NW coast of the island and was constantly inhabited from the 10th century B.C until the 6th or 7th century A.D. A series of sub- and extra-urban sanctuaries have been located. On the acropolis there was a Thesmophorion, while lower down in the crest of the upper town (Middle Plateau), there were a series of sanctuaries and public spaces. One was doubless dedicated to Aphrodite (probably Building 2 of the Middle Plateau). A sanctuary of Apollo and Artemis was excavated at the North extremity of the Middle Plateau, dated from the Archaic to the Hellenistic period. The back room of the Doric temple (Building 3) was full with offerings of precious metals, mostly jewels, and a great variety of luxurious objects, dated mostly from the seventh to the early fifth centuries B.C. The finds in the "adyton" were perhaps re-positioned there during the Hellenistic period, following a damage of the temple by an earthquake. A second violent destruction some time later led to the final collapse of the edifice and the preservation of its contents up to the present day. The double temple with its two altars stood on a massive terrace which on its turn was bordered by a monumental terrace wall, more than 60 metres long. The latter may belong to the Hellenistic period and seems to mark the monumental reorganization of the entire upper plateau of the ancient city, allowing large-scale communal gatherings. It cannot be excluded that the Agora of the Kythnians was situated here. This idea is further strengthened by the discovery and excavation (from 2009 to the present day) a few metres lower down the slope of a large public edifice dated in the late Classical and Hellenistic period The architectural layout and the finds, which include votive offerings and official lead weights and measures of the Kythnians, suggest that the edifice may have been a Prytaneion.
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