Very few things are known for Skiathos during the early
Byzantine era.
We only know that in 325 AD the residents of the island begun
converting to Christianity and that in 530 AD, Skiathos became
an episcopal see belonging to the Metropolis of Larissa and
that the first church, Trinity, was built. Also, in 758 AD the
Byzantine fleet -which aided Thessaloniki when it was about
to be attacked by the Slavs and Bulgarians- was moored in
Skiathos.
The prevalence of Christianity during the 4th c. AD, the raids of the Saracen pirates during the 8th c. AD, the Byzantine Iconoclasm, the uprising of the Aegean islands against the Isaurian dynasty, the destruction of Skiathos by the Emperor’s troops in 758 and the four centuries of “historical silence” that attest to the decadence and the misfortune of the land, broadly make up the background that leads to the era of the Venetian Rule that lasted from 1204 until 1276.
In 1204 the Byzantine empire comes to an end on the hands of the Franks, and the Aegean islands are given to the Venetians. specifically, the Venetian entrepreneurs and brothers Andrea and Geremia Ghisi, take over Skiathos in 1207. They allowed the island to be self-governed and they granted many privileges to the locals, mentioned in the famed “CAPITULA SCIATI ET SCOPULI”. However, they abolished the Orthodox Diocese while building a new castle in the big port to live in and to also aid in the safety of the town, naming it Bourtzi (link). The Ghisi brothers and their successors remained on the island until 1276, when Skiathos and the rest of the Sporades were once again under Byzantine rule.
Skiathos was fought over by the Byzantines and the Venetians until 1390 when a Greek was appointed Commander of the island in the name of Constantinople until 1453. However, the dominance of Constantinople was probably more on paper, since the danger of the Venetians, pirates and later Turks doesn’t allow the Byzantine capital to establish its dominance and forces the people of Skiathos to abandon their seaside town and settle on the north part of the island, the steep rock/natural fortress of Kastro, from the middle of the 14th century and for at least 5 centuries.
Upon the fall of Constantinople, the people of Skiathos pick and ask for the Venetian rule because they realise that, from that point forward, Venice would be the only strong protection against the Turks. They only thing they demand is that the privileges the Ghisi brothers had given to the island remain and that the Orthodox bishop’s see is maintained, demands that were met by the Venetians.
Thus begins the second part of the Venetian Rule that lasted until 1538, except things are much different than before, with extreme oppression of the population. The pirate raids continue and the oppression of the locals is so intense that in 1538, when Kastro was under siege by the Turk Admiral Barbarossa, some locals ended up delivering it to the besiegers to rid themselves of the Venetian tyranny.