Kastro is the old, picturesque main village of Sifnos (Cyclades), built on a steep hill or cliff (approx. 80 m high) on the east coast, overlooking the Aegean Sea. It has been inhabited since prehistoric times (3rd millennium BC) and was the island's ancient capital (the "asty" mentioned by Herodotus).
In the 6th century BC, it flourished thanks to mining, boasting an acropolis, a temple, and a theater dedicated to Dionysus.
Remains of ancient walls, columns, and small Roman sarcophagi can still be seen in the narrow streets.
During the Middle Ages (starting in the early 14th century), the Catalan Da Corogna family fortified the settlement, transforming it into a compact, fortified village featuring two concentric rings of houses.
It remained the capital until 1836, when Apollonia took over that role.
As a result, its medieval character has been remarkably well preserved.
Points of interest include the Archaeological Museum of Sifnos, the Church of the Seven Martyrs (Eptamartyros)—situated on a rocky outcrop jutting into the sea—the old gateways (loggias), and various 16th- and 17th-century chapels.
It is a living open-air museum characterized by narrow, car-free alleyways.
At the foot of the village lies the small bay of Seralia (the old harbor).
