On the Nisi peninsula of Palaia Epidaurus extensive fortifications of Classical antiquity have survived. The medieval castle, although it occupies only a small part of the ancient walled area, still covers a large area. With a perimeter of about 500 m, it comprises an area of some 13,400 sq. m. The enclosure has five sides, with the longest on the east. Remains of three towers are still visible today; it is speculated that there were also another three at the corner-angles in the walls. The enceinte wall is extremely strong, as its thickness reaches up to 2 m. Building material from ancient fortifications and in particular from the nearby theatre was used for the construction of the walls of the new castle. For the most part the walls consist of an outer face built with the squared ancient stone blocks, in between whose joints bricks, small stones and mortar are inserted. The inner face of the walls is just as elaborate. Between the facades are a rubble fill, soil and mortar. In some other places the walls are built with rubble, thus testifying to later alterations.
Inside the castle, near the east wall, loom large the ruins of a single-aisle vaulted church with outbuildings, probably dating between the 7th and 10th centuries AD. To its west stands a modern chapel.