Before the archaeological discoveries in 1990, the region of Poros was mainly known for its significant presence of classical antiquity when Kefalonia was divided into four states Cranes, Sami, Pali and Pronnoi. The area belonged to the Kingdom of Pronnoi city, and consisted of the entire northeastern side of Kefalonia up to the Mediterranean, with the contemporary harbor of Poros easterly and also the regions of Elios Katelios, Skala.   The remain of two powerful acropolis at Pachni hill, modern-day Poros and a second one at Syria castle, the place that nowadays is called PaleoCastro (old castle), a steep summit that observes all the northeastern part of the island, evince the importance of the city of state of Pronnoi, during the the classical era.

    A big part of the fortifying surrounding of the city where the center of the city-state of Pronnoi was situated, according to a theory exist at Paleocastro acropolis until now. The second acropolis with its imposing walls lie at Pachni hill, which is the hillock above the contemporary settlement of Poros. Some people identify this acropolis and  not Paleocastro, with the Capitol of ancient Pronnoi. The acropolis seizes a flat summit, the Ano PAchni (upper Pachni), which is situated southwestely exactly over Poros gulf. From there, the hill slopes northwesterly and shapes a second lower summit over Poros Canyon which is part of the walking trail and is called Kato Pachni (The lower part of PAchni). The fortification includes all this extended area. Near to the northwestern angle of the flat summit of the acropolis exists a gate, the sides of which are built with huge polygonal quarried stones. The south wall slopes towards the side of the canyon, following the declivity of the mountain.

    From time to time researches at the surface, easterly and southeasterly of Pachni have led to the discovery of ceramic pieces dated probably to the prehistoric era. Also remains of houses, prehistoric tools from pyrite and pottery of the ProtoHelladic years (3000-2000 B.C.) have come to light. These finds are important because they prove that in prehistoric years the acropolis was already being used.

    The lumber from the fir of the mountain Ainos, which partly belonged to Pronnoi Kingdom, played a significant role in the development of the area to a very important naval and strategic force from prehistoric until historic years. The history of Pronnoi remains vague in historic times and only in participation in the 2nd Athenian alliance at 445 BC is known. Philip the 4th of Macedonia, abandoned the thought of laying a sige to it, due to its natural fortification. The Hellenistic cemeteries of Pronnoi and the Romaic villas indicate the historic course.

The acropolis of Pronni gave the impression that Poros exists  since the classical period. Yet, the recent discovery and excavation of a great vaulted tomb  of the Mycenean period, which was found at the location Broutzi of Tzanata, indicates that the area was populated much earlier, since prehistoric times. The tomb is the greatest and  the best preserved of those found on the island. Its diameter is 6,80 meters and it is built on  a rocky clearing of a hill. Inside the tomb were found many burials which are dated from  1400 to 1000 B.C. This tomb signals the existence of a mighty Mycenean centre, probably  that of the Homerian Ithaka.  Poros, following the history of the island, had many conquerors: Romans, Franks,  Venetians, Italians, French, Russians,  and finally the British, until 1864 when Kefalonia was united with Greece. In  1821 Napier, the British Governor of the island, afraid that the area would become unpopulated, brought settlers from Malta.  This model agricultural settlement that he intended to create never succeeded.