The main entrance to the port of Spinalonga is protected by a semicircular bastion located at the island's northern edge, the Michiel demi-lune. This is one of the finest and most fascinating examples of a sea bastion preserved among the fortresses of the Most Serene Republic of Venice. Externally, on the demi-lune's base, an inscription is preserved, commemorating the General Provisioner of the Kingdom of Crete (Provisor Generalis Regni Cretae), Luca Michiel, who supervised the commencement of the fortification works, along with the construction year 1579.

In total, it has seven arched cannon port openings at positions deemed suitable for the entrance's control. Later additions include the embrasures and the wall on the square's north side, which destroys the initial access ramp. There was a small house for the garrison on the square.