The Venetian fortress of Koules dominates the entrance to the old harbour of Heraklion. The Venetians called it "Rocca al Mare" (Sea fortress), but today it is known by its Turkish name of Koules (su kulesi). This is one of the most famous and beloved attractions is its symbol.

The exact history of the origin of the fortress is unknown, but the harbour with such a strategic position in the Mediterranean sea could not remain unprotected. The first fortress was probably built on the site of Kulesa in the Arabic period (9-10 century). In some sources there are mentions of the fortress during the Byzantine period (10th-13th century). There are sketches of travelers of that time, the earliest of which are sketches of the fortress monk of Buondelmonti (1429).

In the middle of the 14th century in Europe appeared gunpowder (a mixture of saltpeter, sulfur and carbon). His appearance has played an important role in the war and radically changed military science. There is a need to review the defenses of the former fortifications. Thus, by the mid-15th century, the current castle was weak and useless for the defense of the city object. In 1462, the Venetian Senate approved an ambitious program of strengthening of Heraklion and surrounding areas. In this project, the old fortress of the harbour, which by that time was thoroughly damaged by the earthquakes and the destructive power of the sea, was demolished (1523), and in its place was built a new building, which survived until our days. The work lasted until 1540.

The fortress was erected on the platform formed by the natural rock ledges. The building has two floors with 26 rooms and covers an area of 3600 sq. m. the thickness of the outer walls 9 meters, the interior walls in some places reach 3 meters. There were three entrances to the fortress from the West (main entrance), Northern and South-Western sides. The exterior walls were decorated with different signs, inscriptions and coats of arms. The entrances were decorated with marble reliefs depicting winged lion of St. Mark (the symbol of the Venetian Republic). Two of these reliefs have been preserved to our days.

The first floor housed the jail and the room where he kept stores of food and ammunition. There were also separate rooms for soldiers, officers and the Governor. The fortress had the mill, the oven and the chapel that provided her autonomy. On the North side of the upper floor was a lighthouse. During the reign of the Turks was completed the upper level of the fortress, added embrasures for cannons and small mosque.

Today the fortress of Koules is sometimes used for art exhibitions which are held in a room on the first floor. On the upper floor hosts concerts and performances, weather permitting.