The visitor of Kythnos has the opportunity to see many archaeological sites of significance. One of these sites, still being excavated, is Vryokastro, thought to be the ancient capital of the island.

Vryokastro is located on the northwest side of the island, between the bay of Merichas and the bay of Apokrousis. The ancient city was called Kythnos from where the name of the island today is derived. According to the findings, it has been inhabited between the 10th century BC. and the 7th century BC and after that it was abandoned by its inhabitants, who moved to the Castle of Oria. The excavations carried out between 1990-1995 and in 2001 revealed findings and data testifying the housing and urban development of the ancient capital of Kythnos. The excavation that started in 2002 was done by the University of Thessaly under the direction of Professor Mazarakis Ainian, in collaboration with the Department of Prehistoric, Classical and Ancient Antiquities.