Kythnos has a rich history and an abundance of interesting archaeological sites.
One of the most important archaeological sites of the island is located in the northwest and is identified as the ancient capital of the island. The archaeological findings are rich and visitors will soon be able to admire them in the new archaeological museum in Kythnos. Two of the most important excavations are the sanctuary of Demeter at the top of Acropolis and the underground water tanks used for the collection of water from rain. The area occupied by the archaeological site is about 385 acres and includes the rocky island of Vryokastraki, which at the time was connected to the island of Kythnos. Vryokastro was abandoned in the 7th century AD. when the inhabitants moved to the Castle of Oria.
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The archaeological site of Maroulas is located on the northeastern side of the island near the village of Loutra. It is the only open air settlement of this period that has been found in Greece. There have been excavated circular residences and graves. Most likely it was a base station or a refueling station.
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Located in the northern part of the island, very close to Chora, it used to be the capital of the island during the Middle Ages (from the 7th to the 16th century AD). Formerly it was called the Castle of Thermae, the old name of Kythnos. But the place seems to have been inhabited since prehistoric times. Nowadays, only the ruined walls and some residences are visible, while the churches of Agia Triada and Karaleousa are in good shape.
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One of the most important sites of the early cycladic period is located on the steep northeastern side of the island. It dates back to about 3,000 BC. So far, 20 circular structures have been discovered made from stone. The site includes mining furnaces, as well as a copper mine at cape Juli. These excavations give Kythos a leading role in the development of early metallurgy.
On the island of Kythnos there are about 30 towers scattered all around the island. They are of rectangular and circular shape and chronologically they are placed in the classical period. The most well-known are Pyrgos near Chora and Paliopyrgos neat Aspra Spitia or Aspra Kélia.
In the southwest part of the island, near Kastella beach, an ancient acropolis has been discovered. Unfortunately, only the ruins remain on a nearby hill. The excavation showed that it was founded in early historical times and was abandoned at the end of the Archaic period.