Marco Sanudo was a nephew of the Venetian Doge Enrico Dandolo and a participant in the Fourth Crusade. With his keen interest in the island of Naxos, he arrived at the harbor of Potamides in 1207, along with eight galleys loaned to him by the Venetian arsenal. The locals put up a fair fight, but Marco Sanudo successfully captured the Greek fortress of Apalyros after a five-week siege. Slowly moving forward, he managed to capture the entire island of Naxos as well as other Cycladic islands by 1209 and inaugurated himself the Duke of the Duchy of the Archipelago in 1210, with Naxos as its capital.
Marco Sanudo fortified Naxos and divided it into 56 provinces. He introduced a feudal system according to which each province was distributed and assigned to those who had helped him in his crusade as well as those who came in from Venice to fulfill administrative duties. This system was easy to introduce as the local Greeks were already used to the Byzantine pronoia system.
He gave away most of his captured islands to those who had assisted him in his endeavor as fiefs. His cousin Marino Dandolo received Andros, Andrew and Jeremiah Ghisi received Tinos, Mykonos and the Sporades, John Querini acquired Astypalea, Jacob Bertozzi received Thera, and Leonardo Foscolo received Anafi. Other islands, including Naxos, Paros, Milos, Sifnos, Kythnos, and Syros, remained under Marco's power.
Naxos was one of Marco Sanudo's favorite islands. He played a pivotal role in its subsequent history and culture. He made Naxos the administrative center of his duchy and remained the state's center until it fell under Ottoman rule in 1566 AD. Sanudo, with the help of a group of trained engineers, built a wonderful specimen of medieval architecture, the Castle (Kastro).
The Castle was essentially a walled city within a pentagon-shaped defensive wall. Marco chose the highest point above sea level, a site of an ancient citadel, to build the Castle. At the center lay a Catholic cathedral and the Cagellaria (or administrative tower). The nobles usually built their houses here. There were five lookout towers originally, out of which only one remains today. The Castle had three entrances, one to the north, one to the south, and a third to the east.
Marco Sanudo died in 1227, and his reign was followed by that of twenty-one dukes from the Sanudo and Crispo dynasties. After his death, his descendants invited locals to come and build houses near the Castle, establishing a new neighborhood that was called the Bourgos. As the Duchy expanded and the need for space grew, more and more houses were built, with the Castle being the center of this urban development. Marco Sanudo was a wise ruler and truly cared about the well-being of his subjects even though he was from a foreign land. He had rightfully earned love and respect from both Venetian and Greek people.