Kastri, in the north of the island near the village of Chalandriani, is one of the most important Early Bronze Age sites in the Cyclades. The ruins of a fortified settlement occupy a rocky promontory, and the site is associated with a large adjacent cemetery that yielded many of the Cycladic figurines now in the Archaeological Museum. The views from the hilltop are superb.
The site is not heavily signposted and the ruins themselves require some imagination — you are looking at low stone walls and foundations rather than standing monuments. But the setting is remarkable and the context, once you understand what was here 5,000 years ago, is quietly impressive. Rent a scooter or hire a taxi from Ermoupoli; the site is not served by public buses. Bring water — the hilltop is exposed.