An emblematic monument of Heraklion is the Venetian sea fortress at the tip of the north-western harbour mole, known by its Ottoman name Kules (Kule, “tower”), which replaced the Venetian Rocca or Castello a Mare. Originally, the site was occupied by the Castellum Communis, likely a beacon tower from the Hellenistic and early Byzantine harbour fortifications, retained by the Venetians with minor modifications.

The construction of the new fortress was commissioned in 1523 to meet the requirements of artillery and to provide effective protection for the harbour and coastal defences. For the construction of the enlarged foundation, ships loaded with rocks and stones were sunk into the seabed. The fortress has a quadrilateral plan with a pronounced semicircular projection on the south-eastern side. The main entrance opens on the western façade, at the end of the Venetian fortified mole.

Inside, barrel-vaulted chambers accommodated garrisons, prisons, cisterns, and storerooms. Secondary halls featured raised timber floors and, in some places, wooden mezzanines. Cannon ports were arranged along the ground-floor perimeter, while a large staircase (1539) and a sloped passage for hauling artillery led to the roof, originally laid out as a broad terrace with recessed cannon ports, an elevated walkway, and a straight parapet with a cordone. A lighthouse tower stood on the northern side, and the fortress also housed living quarters for the garrison and officers, a mill, and a bakery.

On three façades dominates the relief emblem of Venice, the winged Lion of Saint Mark. The earliest was placed in 1533 above the central gate of the western façade, accompanied by an inscription naming Doge Andrea Gritti and senior officials. Around the same period, the lion of the southern façade was installed, now headless, while the emblem on the north-eastern side, featuring the figure of the “Eternal Father” in the pediment, dates to 1534.

The fortress constantly suffered from the destructive force of the waves, necessitating repeated repairs by the Venetians until the onset of the siege, during which its defensive role was swiftly neutralised. Following the fall of Candia, the Ottomans carried out repairs, added battlements, and converted the lighthouse tower into a small mosque with a minaret. The fortress was renamed “Great Kule” or Water Fortress (Su Kulesi). At the end of the leeward mole, the “Little Kule” was constructed, which was demolished in 1936 during the development of the modern harbour.

From the 1930s onwards, modern interventions were undertaken to protect and repurpose the monument, with a comprehensive restoration and enhancement carried out by the Heraklion Ephorate of Antiquities between 2011 and 2016.