Kastro, the fortified pirate-proof medieval capital of the island, is perhaps the most interesting site in Skiathos. The Greeks made a castle during the 15th century to take refuge from marauding pirates. It has been historically proven that when the Venetians took possession of the island for the second time in 1453, the only town on the island was inside the Kastro. It seems likely that the Kastro was built in the mid-14th century, when the Saracens and other pirates roamed the seas and often landed on the islands and ravaged them.

How to reach Kastro. All the island excursion boats call here. However, the Kastro can be reached on foot, a two-hour hike from the port. Excerpts from "Trekking in Greece" by Marc Dubin (Lonely Planet, 1993):

Walk north from the dock of Skiathos harbour on the asphalt road towards the airport. Some 15 minutes out of town, the road bears sharply right, and you should veer left onto a dirt track prominently marked by red dots on a tree, a 'W' (perhaps for German weg) and, most obviously, a multicoloured sign reading 'Panagia Evangelistria'. Climb along this jeep track for another quarter of an hour until a wooden cross and a white and green ikonisma mark the actual start of the trail (the track going straight goes to Evangelistrias monastery). Turn left onto the footpath and proceed an occasionally steep 15 minutes more on a well-worn surface to another white and green ikonisma marking a confusing intersection. Here red dots and a sign seem to suggest you take a left fork, but the right turning actually works better. About an hour from town you'll reach the fountain of Agios Dimitrios, identified with a plaque. Within another 10 minutes you'll meet a new logging road; turn right (north) and walk a few paces to the point where the trail resumes on the far side. Soon cobbling appears and the route enters shade; some 20 minutes above the road, ignore an uncobbled fork on the right and pass an unreliable cistern recessed into the hillside on the left of the path. Just 90 minutes from Hora, the kalderimi is briefly disrupted by another bulldozer track in the vicinity of a red and white ikonisma. Cross the road-disrupted saddle, aiming for the ochre ksoklisi of Agios Apostolos, set at the 433 metre summit of the island just north of the path's continuation. In the next 15 minutes the cobbles resume and dip down to meet another dirt road on its way to the north coast. Turn right and walk a few paces to yet another ikonisma on the right verge, painted with the word 'KASTRO' and a red arrow pointing to the onward path. Within 15 minutes of leaving the road on this path, keep going straight towards the chapel of Panagia Kardasi. Here the belfry has the form of a ship' s mast with a crow's nest, and glass fishing floats whimsically stud the roof line. Within another 15 minutes you'll reach the well maintained grounds of Agios loannis monastery, complete with surging fountains, flower beds and picnic tables. Another 10 minutes, or 2 1/2 hours from the harbor  brings you to the entrance of Kastro.

Kastro is more a natural fortress than a man-made one. Three of its four sides overlooking the sea were surrounded by walls with embrasures. In the past, the only access was by a wooden drawbridge. In times of danger, the bridge was drawn back into the fortress and the enemy was thus unable to pass through. The defendants dropped hot oil on any attackers who might have succeeded in reaching the iron gate.

The houses in the Kastro were built close together and most were small and dark. Inside the fortress there were also water cisterns and several churches - over 20, according to tradition. From the period of Turkish occupation on, there was also a mosque, without a minaret. Today the ruins are largely overgrown, and only two churches survive intact, with fine icons and some frescoes. Christos, a single-aisled basilica, with walls made mainly of rough stone, used to be the main church of Kastro. It was restored from time to time. Part of the wall and gate of the old fort, as well as half destroyed mosque are still visible but nothing remains of the fort's tower and the residence of the governor built during the Venetian occupation. The drawbridge is now ruined and is replaced by cement stairs.