Also known as the Old Town, Kastro perches on a forbidding promontory high above the water, accessible only by steps. Skiathians founded this former capital in the 16th century when they fled from the pirates and the turmoil on the coast to the security of this remote cliff—staying until 1829. Its landward side was additionally protected by a moat and drawbridge, and inside the stout walls they erected 300 houses and 22 churches, of which only 2 remain. The little Church of the Nativity has some icons and must have heard many prayers for deliverance from the sieges that left the Skiathians close to starvation.
You can drive or take a taxi or bus to within 325 feet of the Old Town, or wear comfortable shoes for a walk that's mostly uphill. Better, take the downhill walk back to Skiathos town; the trek takes about three hours and goes through orchards, fields, and forests on the well-marked paths of the interior.