The Castle of Neratzia guards the harbor of Kos opposite the coast of Asia Minor. Earlier Byzantine defenses and buildings occupied the area before the Knights Hospitaller established authority on the island in the fourteenth century. The knights developed an inner stronghold and later a larger outer enclosure, using the harbor castle as part of a defensive network centered on Rhodes. Its position protected shipping, supervised the town, and connected Kos to fortified stations that supported communication and resupply across the southeastern Aegean.

Construction continued through the fourteenth and fifteenth centuries, with major work after earthquakes and amid growing Ottoman pressure. Builders reused columns, blocks, and inscriptions from ancient Kos, embedding the classical city within medieval walls. The inner castle provided the earlier core, while the broad outer ward added towers, gun positions, gates, and space for soldiers and stores. Coats of arms of Hospitaller officials recorded phases of patronage. The resulting complex was not a single design but an accumulation of repairs adapted to artillery and changing threats.

The Ottoman conquest of Rhodes in 1522 made the Hospitallers' position on Kos untenable, and Ottoman authority followed soon afterward. The castle served the new rulers as a garrison and harbor defense. Mosques, houses, magazines, and military structures in and around the enclosure altered the medieval setting, while earthquakes repeatedly damaged the island. The fortress remained a prominent point of control even as improvements in naval warfare reduced the effectiveness of its walls against modern ships and guns.

Italy seized Kos in 1912 during war with the Ottoman Empire and governed the Dodecanese until the Second World War. Italian archaeologists and administrators cleared, studied, and restored ancient and medieval monuments, often making choices that reflected colonial ideas about which historical periods to emphasize. Wartime occupation and the transfer of the islands to Greece in 1947 added further political layers. The bridge and harbor setting continued to bind the castle visually to the ancient agora and the modern town.

A strong earthquake in 2017 damaged the monument and forced closure while engineers and conservators assessed walls, gates, and visitor routes. Subsequent restoration stabilized vulnerable masonry and allowed renewed access. The work highlighted the continuing seismic risks that have shaped the castle since its construction. Neratzia now preserves evidence of Byzantine occupation, Hospitaller military organization, extensive ancient spolia, Ottoman use, Italian-era archaeology, and modern Greek conservation. Its history is inseparable from the harbor, whose strategic value repeatedly drew imperial powers to the same small piece of ground.