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

The exact origin of the history of the fortress is unknown, but the harbor with a strategic position in the Mediterranean could not remain unprotected. The first castle was likely built on the site Kulesa still in Arabic period (9-10 century). Some sources have mentioned fortress in the Byzantine period (10-13 centuries). There are sketches of travelers of the time, the earliest of which are sketches of the fortress monk Buondelmonti (1429).

In the middle of the 14th century in Europe there was gunpowder (a mixture of saltpeter, sulfur and carbon). His appearance played an important role in the war and radically changed the military science. There is a need to revise the defenses of 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 to strengthen Heraklion and nearby areas. Under this project, the old fortress of the harbor, which by then thoroughly affected by the earthquake and the destructive power of the sea, was demolished (1523), and in its place was built a new building, which also survived to this day. The works lasted until 1540.

The fortress was built on a platform formed by natural rock ledges. The building has two floors with 26 rooms and covers an area of ​​3,600 sq.m. The thickness of the external walls of almost 9 meters, interior walls in some places as high as 3 meters. There are three entrances to the castle from the west (the main entrance), northern and south-western sides. The outer walls were decorated with different signs, inscriptions and coats of arms. Entrance is decorated with marble reliefs depicting the winged lion of St. Mark (symbol of the Venetian Republic). Two of these reliefs have survived.

The ground floor housed a prison and the premises where stored food supplies and ammunition. There were also separate rooms for soldiers, officers and the governor. The fortress had a mill, a furnace and a chapel, which provides it with autonomy. On the north side of the upper floor is a lighthouse. During the reign of the Turks was completed the upper level of the fortress, added loopholes, place for guns and a small mosque.

Today the fortress Koules is sometimes used for art exhibitions, which are held in a room on the first floor. The top floor hosts concerts and performances, if the weather permits.