Palaiokastro or Paleokastro or Paliokastro or Palionavarino is a medieval castle and an ancient acropolis, on top of a steep cliff overlooking the Gialova lagoon, at the north side of the bay of Navarino. There were two important castles in this bay. The other one is Niokastro. The ancient acropolis of Pylos was at the same location. This was not the Pylos of Nestor and of the epic poems of Homer, but the Pylos of the classic period which played a role in the Peloponnesian war.
The medieval castle was built in 1278 by Nicholas II of Saint Omer the French feudal ruler of Thebes, who had become Baron of Kalamata after his marriage to Angelina Komnene, widow of the Prince of Achaea Guillaume II de Villehardouin. The initial name of the castle was Port de Jonc (Bulrush Port).