The Kastro is the fortified hilltop quarter that Marco Sanudo, the first Duke of the Archipelago, built after the Fourth Crusade delivered the Aegean to Venetian hands in 1207. Sanudo chose Naxos as the capital of his new island duchy, and the walled town he raised on the hill above the port lasted — with modifications — for the next three centuries. What you walk through today is one of the best-preserved medieval settlements in Greece, inhabited without interruption since the 13th century.
The approach from below involves a climb through the Bourgos neighbourhood, the lower old town, before passing through one of the original fortified gates into the walled interior. Inside, the lanes narrow and the light changes — these streets were built for foot traffic and shade, not for cars or tourists. The Venetian tower houses (pyrgoi) that line the main lanes were built by noble families according to a strict feudal hierarchy: the taller your tower, the more powerful your family. Several survive intact, including the towers of the Sanudo, Crispi, and Barozzi families, their stone coats of arms still visible above the doorways.
The square at the heart of the Kastro is small and shaded by a few trees, with a Catholic cathedral (the Latin church of Naxos's long Venetian past) and the building that now houses the Archaeological Museum. It feels genuinely lived-in: washing hung between buildings, a cat on every other step, locals going about their day in near-complete indifference to the visitors threading through. The effect is much more authentic than the carefully preserved medieval quarters of more visited islands.
Come in the morning when the light is best for the stone and before the cruise-ship groups arrive. The lanes beyond the main circuit are worth exploring — you'll find small chapels, collapsed walls giving out over the rooftops and sea, and the occasional café tucked into an improbable corner.