At the southern summit of the hill named Nissi stands imposingly the castle of Paralio Astros, the construction of which dates to the Frankish rule. It is a medieval fortress, which slowly developed into a powerful defensive complex, with a significant role in various armed conflicts, not an uncommon phenomenon during those times.

Near the castle is also the historical location of Geneon Pedion, mentioned also by Pausanias. There, in 546 B.C. took place the most important battle between the Argeians and the Lacedaimonians, instigated by the controversial matter of who would prevail in Kynouria. It was also called battle of Thyrea or battle of the 600 logades (= the chosen ones).

At the castle, which can be visited throughout the year, one can see remnants of various buildings and constructions, the use of which remains unknown to this day. The ones for which we do have information are the three houses built in the interior by the Zafiropoulos brothers, who had returned from abroad in order to help out in the Greek Struggle for Independence.

The Castle of Paralio Astros is situated on the top of Nissi hill, which towers over the port. It was built in 1256 AD by Prince William Villehardouin, who called it Castello della estella owing to the star shape of the peninsula that inspired the name of the region; (Astros meaning star). This is a castle with a four-sided floor plan, with one of the towers and part of the fortification surviving.

There is evidence of activity during ancient times in the area around the castle. Remains of a wall from the classical period are visible on a hill, which the Aiginites started to build but did not complete. The area has also been identified as the so-called Geneon Pedion, where according to Pausanias, the most important battles between the Argives and the Lacedaemonians took place, for the conquest of the Valley of Thyrea and for the general dominance of Kynouria.

The castle also played an important role during the period of Turkish occupation as a powerful centre for defence. There is reference of a charge by Ibrahim that was successfully repelled. During the Greek Revolution, the brothers Zafeiropouloi, rich merchants from abroad, decided to return home and participate actively in the struggle for the liberation of Greece. They chose the castle as the place where they would set up base, which they repaired, while they built their three residences in its interior. The first, the residence of Panos Zafeiropoulos (whom they called akouros because he had sworn not to cut his hair and beard until the homeland was free) has a rectangular floor plan and is built on the foundations of an older building. In the second, that of Konstantinos Zafeiropoulos, we see four rooms, auxiliary spaces, a rest room and cisterns. The third residence, belonging to Ioannis Zafeiropoulos, is in a much worse state that the first two, but is quite similar to the residence of Konstantinos.

Restoration works have been carried out on the castle today, and one can visit it throughout the year to enjoy the magnificent views it offers.