Neratzia, Medieval fortress at the harbor entrance in Kos, Greece.

Neratzia is a medieval castle at the harbor entrance of Kos, built on a small promontory that separates the port from the open sea. It has two concentric enclosures, one inside the other, each reinforced with round towers at the corners and thick stone walls facing the water.

The Knights Hospitaller took over Kos in 1314 under Grand Master Foulques de Villaret, and construction of the fortress began around 1377 under Jean Fernandez de Heredia, using stone from the ancient city and the Asklepieion. A first written mention dates from 1395. An outer ring with corner bastions was added from about 1495 and finished in 1514 under Fabrizio del Carretto; the fortress was handed to Ottoman troops on 6 January 1523.

Granite columns and carved stone fragments taken from ancient Greek buildings are built directly into the outer walls, visible to anyone who walks along the base of the fortification. These recycled pieces sit side by side with the rough medieval stonework, making the layering of time visible without any explanation needed.

The only access is a stone bridge from Platia Platanou, crossing the former moat that is now Leoforos ton Finikon, the palm-lined avenue. The site closed to visitors in July 2017 after earthquake damage and reopened partially on 14 May 2021, so some sections may remain off-limits. The island's ferry port lies at the northern end of the same peninsula.

The oldest tower inside the fortress carries coats of arms from several different Grand Masters of the Knights Hospitaller, each one marking a separate building phase. By following them in order, a visitor can read the construction history of the site directly from the stone.