High on a rock above the sea on the northwest end of Kythnos is Kastro Horias, the capital of the island during Byzantine and Venetian rule. Said to be impregnable the town is in ruins now but the ancient walls and streets are still in evidence and there are two of the 100 churches that used to stand here, that have been restored. You can reach Kastro Horias, also known as Kefalikastro, by car or on foot.
Besides Kastro, the most important archaeological sites of Kythnos include Vryokastro, the ancient 10th century B.C. capital on the norteastern coast. There are the remains of some walls and towers and two Hellenistic temples. Little remains of the ancient site of Kastellas but the site of Maroulas, perhaps the oldest site in the Cyclades, has the remains of some buildings from the 8th and 7th Century B.C.