Mount Fokas is a 862m high hill in in the area of Corinth. A medieval settlement existed on its trapezoid summit and later a castle was built by the Franks in the 13th century (or perhaps by the Byzantines, a little earlier).
The castle controlled the plains of Nemea and the main route to the center of Peloponnese. It had visual contact with other Frankish castles in the area like Acrocorinth or Agios Vasilios but also with some castles in Central Greece, on the opposite side of the gulf of Corinth.
Only a few ruins of the walls and from the settlement remain at the lower part of the rock.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.