The castle of Kyparissia stands on top of a steep hill at the north-western edge of Ano Poli (the upper city), at the site of the ancient acropolis. Built on a naturally protected location, it had control of the passes from the western Peloponnese to the fertile plains of Triphylia and Ano Messenia. In the lowland area northwest of the castle until the coast of the Ionian Sea and within the modern city, building and burial remains have been excavated from time to time, testifying to the history of the city during Hellenistic and Roman times. The castle dates to the byzantine period. Its core was built with re-used ancient material. In 1205, when Kyparissia was named Arcadiá, due to the large number of people from Arcadia that had settled there during the Slavic raids (7th-8th centuries), it was conquered by the Franks. Along with the castle of Kalamata, it was a personal feud of the Villehardouins. After 1430 it was part of the Despotate of the Morea, until its conquest by the Ottomans in 1460.

It has a square plan, following the contour lines. A cross wall was dividing the outer (south) bailey from the inner (north) one. The vertical walls were interrupted by square towers. After the introduction of fire guns, they were reinforced with a horse-shoe shaped bastion at the north-eastern corner of the inner bailey (15th-16th century) and with a talus (a sloping face at its base) and gun loops in the cross wall. The settlement of Arcadiá stood outside the castle, but in 1668 the Ottoman traveller Evliya Çelebi reported 80 houses inside the castle. A square building, by the main gate at the outer bailey, roofed with a semi spherical vault, has been identified as a small mosque.