A fort built in the sea that is known by its Turkish name "Bourtzi" (burc meaning tower), is the rarest monument of Nafplio. The Italian engineer Antonio Gambello made a clever and strategic move in 1473. He transformed the islet of Aghioi Theodoroi in the bay of Nafplio into a fort. The Venetian Pasqualigo, its "proveditore" (military administrator) fortified it and named it "Castello dello Soglio". This fort in the middle of the sea would make the character of the city more intriguing. From Bourtzi until the port's breakwater, there was a narrow passage isolated with a chain to secure the safety of the port from enemy vessels. Hence, the name Porto Cadena (port of the chain). The ruler of the town was the one who controlled Bourtzi. In 1715, the Turks reinforced it with the so called "porporela": an underwater dam built with stones that obstructed the approach of large ships.
During the War of the Greek Independence, Bourtzi was occupied by the Greek army. From there they would fire cannonballs at the Turkish forces of Nafplio. But also during the internal conflicts that followed the first years of the Greek state, Bourtzi had protected the Greek government twice. Later, when the castles were converted into prisons, it was used as a residence of the executioners who were not allowed to live in the city because the people of Nafplio hated them.
The fort's design is adjusted to the elongated shape of the islet. In its centre, there is a hexagonal three story tower where the communication between floors took place via mobile ladders for security reasons. The water supply was covered by a spacious circular reservoir situated at the basement of the tower. To the north-east a small harbour was designed to provide access to the fort; today the tourists' boats anchor here. The fort was modified and renovated but never lost its special characteristics. Bourtzi is unique, the only sea fort that exists, and the most recognizable landscape in Greece.