Only two of the kástro's original seven towers remain, though the north gate (approached from Apóllonos and known as the Traní Portá or "Great Gate"), by the Kríspi (Glezos) tower, survives as a splendid example of a medieval fort entrance. A few of the Venetians' Catholic descendants still live in the old mansions that encircle the site, many with ancient coats of arms above the doorways. One mansion next to the Traní, the Venetian Museum (Domus Della-Rocca-Barozzi; daily 10am–3pm & 7–10pm (to 11pm in July– Aug); €5, student €3), is open to the public and offers the best views from the kástro; the guided tour includes a tasting from the family's wine cellar; live classical and Naxian evening concerts take place all year in the building or adjoining courtyard. They also offer a two-hour tour of the kástro at 11am Tues– Sun in season; €15 includes entrance to all the kástro museums. Catholic buildings in the kástro include a seventeenth-century Ursuline convent, and the cathedral, unsubtly restored but still displaying a thirteenth-century crest inside. Nearby was what was to become one of Ottoman Greece's first schools, the French Commercial School; opened by Jesuits in 1627 for Catholic and Orthodox students alike, its pupils included, briefly, Nikos Kazantzakis. The school building now houses an excellent archeological museum (Tues– Sun 8.30am–3pm; €3), with finds from Náxos, Koufoníssi, Kéros and Dhonoússa, including an important collection of Early Cycladic figurines. Archaic and Classical sculpture and pottery dating from Neolithic to Roman times, as well as obsidian knives, spectacular gold rosettes from a tomb, and a large collection of Roman glass, are also on display, though labelling is sparse. On the outdoor terrace, a Hellenistic mosaic floor shows a nereid (sea nymph) surrounded by deer and peacocks. In the centre of the kástro are the plain stone remains of a rectangular tower, said to be the residence of Sanudo.

More archeology is accessible a few minutes' walk east from the harbour, at the remains of the Mycenaean city. In front of the Orthodox cathedral, the Mitropoleos Museum (Tues– Sun 8.30am–3pm; free) has walkways over a recently excavated tumulus cemetery, from the thirteenth to the eleventh century BC, with various funerary remnants including a hermax, a pile of the stones that were traditionally thrown behind on leaving a cemetery. In the general area, items dating from the early Cycladic period (3200 BC) right through to late Roman (500 AD) have been found.