In 1578 the Venetians charged the engineer Genese Bressani to plan the island's fortifications.
He created blockhouses at the highest points of the northern and southern side of the island, as well as a fortification ring along the coast that closed out any hostile disembarkation.
In 1579, the Provveditore generale di Candia, Luca Michiel, put the foundation stone of the fortifications, built over the ruins of an acropolis.
There are two inscriptions that cite this event, one on the transom of the main gate of the castle and the other on the base of the rampart at the north side of the castle.
In 1584, the Venetians, realising that the coastal fortifications were easy to conquer by the enemies attacking from the nearby hills, decided to strengthen their defense by constructing new fortifications at the top of the hill.
The Venetian fire would thus have bigger range, rendering Spinalonga an impregnable sea fortress, one of the most important in the Mediterranean basin.

Spinalonga, along with Gramvousa and Souda, remained in Venetian hands even after the rest of Crete fell to the Ottomans in the Cretan War (1645–1669) and until 1715, when they fell to the Ottomans during the last Ottoman–Venetian War.
These three forts defended Venetian trade routes and were also useful bases in the event of a new Venetian-Turkish war for Crete.
Many Christians found refuge in these fortresses to escape persecution from the Ottoman Turks.
In 1578 the Venetians charged the engineer Genese Bressani to plan the island's fortifications. He created blockhouses at the highest points of the northern and southern side of the island, as well as a fortification ring along the coast that closed out any hostile disembarkation. In 1579, the Provveditore generale di Candia, Luca Michiel, put the foundation stone of the fortifications, built over the ruins of an acropolis. There are two inscriptions that cite this event, one on the transom of the main gate of the castle and the other on the base of the rampart at the north side of the castle. In 1584, the Venetians, realising that the coastal fortifications were easy to conquer by the enemies attacking from the nearby hills, decided to strengthen their defense by constructing new fortifications at the top of the hill. The Venetian fire would thus have bigger range, rendering Spinalonga an impregnable sea fortress, one of the most important in the Mediterranean basin.
Spinalonga, along with Gramvousa and Souda, remained in Venetian hands even after the rest of Crete fell to the Ottomans in the Cretan War (1645–1669) and until 1715, when they fell to the Ottomans during the last Ottoman–Venetian War. These three forts defended Venetian trade routes and were also useful bases in the event of a new Venetian-Turkish war for Crete. Many Christians found refuge in these fortresses to escape persecution from the Ottoman Turks.
Spinalonga