The Fort of Koules ( the word means "fort" in turkish) overlooks the Venetian harbour. It was built by the Venetians in 1211,  as soon as they took possesion of Megalo Kasto, as it was called Heraklion in Byzantine times.  Initially,the fort was called "Rocca al Mare" ("rock at the sea"). 

The construction of Koules anticipated the construction of new walls, as the protection of the harbour was much more urgent. It collapsed by the strong earthquake of 1303 and was rebuilt during the years 1523-1540, when it got its final form. During the Ottoman domination (1669-1898) it was used as a prison and many Cretan heroes of the revolts against Ottomans have lost their lives in the dungeons.

In what concerns its architecture, it is a typical Venetian fort, protecting the breakwater of the harbour; it is the end of the coastal walls. Constructed with huge stones, it is a two-storeyed building. The ground floor has a vaulted roof with broad skylights. Thick walls dived it in 26 appartments, which served as the dwelling of the "Kastelanos" ( the officer responsible for the castle), of the captains and other officers, whereas a part was used as strerooms. The upper floor as well as the basis of a minaret were added to the original construction by the Ottomans.

The castle has been restored and cleaned up, during the last few decades, by the 13th Ephorate of Byzantine Antiquities of the Greek Ministry of Culture.

The place has housed the International Conference about Domenico Theotokopoulos ("El Greco"), which took place along with the great exhibition of paintings of this important and famous artist of the Renaissance. Several cultural events and exhibitions are organised there ever since. A small theatre has been established at the upper floor.

Due to its place, its history and the actual use it has, this archaeological site attracts many visitors all year round.