Koules or otherwise Castello a Mare or Rocca a Mare is the Venetian fortress (tower) that dominates the entrance to the Venetian harbor and its magnificence causes awe to visitors. The Venetians called it the "Sea Fortress" as it protected them from pirates and other sea travellers. It outer walls are 9 meters thick while the inner ones reach up to 3 meters thick. 

Also, the name “Koules” comes from the Turkish Su Kulesi and is one of the most talked about sites of Heraklion and symbol of the city. It is said that in the basement of the tower, the Cretan rebels were held prisoners and tortured.

Today the building is occasionally used for art exhibitions and events, but you can visit Koules and admire its grandeur every day.