Thalassopyrgos or Kasteli according to the Venetians and Bourtzi according to the Turks, is a small islet in front of the port of Nafplio, just opposite the Penthei-Brotherhood artillery fireplace, completely covered by old Venetian fir her name.

This fort was built by the Venetians after Mahmut Pasha`s departure in 1473, supplying it with newer cannons. In 1502 the Venetians, fortifying the southwestern side of Acronafplia into self-propelled bastions (the Five Brothers), connected him with an artificial block of boulders bound by a chain that reached the city of Lyaplaus and reached Thalassopoulos. , hence the name Port of Alysos.

After the Treaty of Karlovych (1698), the Venetians erected a powerful tower and cannon bases on the islet, creating the famous fortress that now dominates the entrance to the port of Nafplio.

During the Greek Revolution of 1821, Thalassopyrgos was captured, in 1822, by 50 gunners and 150 gunners, led by Hastings, Anerman, Hanek, and Dimitrios Kallergis. They were led by French Major General F. Giurden, who regrouped Nafplio from the Thalassopyrgos and managed to thwart the smuggling food of the Turks besieged by an English ship.

In the immediate following decade, during the bloody Greek civil war (1823 – 1833), the then government was twice forced to resort to Thalassopyrgos for its security on May 25, 1824, and July 2, 1827.

After the arrival of King George I and on his orders, in 1865 the Thalassopyrgos was disarmed and became the place of residence of the guillotine keeper.

Finally, with the creation of the Tourism Organization, Thalassopyrgos was renovated at the expense of its private and future landlord and manager, Panagiotis (Takis) Kostourou, and thus converted into one of the first worthy tourist centers in Greece, a Category A hotel. Only 12 rooms and a restaurant with special mention to the best international travel guides of the time. It operated continuously from the 1930s until the years of the April dictatorship, which was considered appropriate to close, without renewing the lease. Footage from his hotel career can be seen in the 1958 movie The Train Man.