The imposing fortress of Heraklion, stands for over 500 years in the city’s obverse. It is the prominent symbol of Venetian Chandax and the landmark of Heraklion historical identity. It was called "Castello a Mare" or "Rocca a Mare" by the Venetians which literally means “Sea Fortress”. The name "Koules" is the alteration of the Turkish word "Su Kulesi" which means "Water Tower". It was constructed for fortification reasons, against the pirate attacks, as well as other conquerors.
The first fortified edifice was placed between 9th -10th. century. Since then, references and sketches by travellers, made their appearance, having as common fact the imposing fortress. European new fortification needs was the main reason of fortress rebuilding. Thus, in the mid of 15th century, Venetians started an ambitious programme of city’s' fortification.
The construction of the large fortress started at the early 16th. century and it was completed on 1540. Since the new building was much larger, the existing rocky foundation had to be enlarged. To achieve that, old ships were loaded with a large amount of limestone and reused material of the Hellenistic fortifications and the nearby lime quarries. The fortress is formed by two sections, the rectangular and the semi-elliptical section and two floors with 26 rooms. The famous three marble compositions with the Winged Lion, the symbol of Saint Mark, the patron of Venice were built into the facades of Koules, accordance with inscriptions reveal Venetian honorable personages.