Mystras, the ‘wonder of the Morea’, developed down the hillside from the fortress built in 1249 by the prince of Achaia, William II of Villehardouin, at the top of a 620 m high hill overlooking Sparta.
The Principality of Achaea was one of the three vassal states of the Latin Empire which replaced the Byzantine Empire after the capture of Constantinople during the Fourth Crusade. It became a vassal of the Kingdom of Thessalonica, along with the Duchy of Athens, until Thessalonica was captured by Theodore, the despot of Epirus, in 1224. After this, Achaea became for a while the dominant power in Greece.
The Franks surrendered the castle to the Byzantines in 1262, it was the centre of Byzantine power in southern Greece, first as the base of the military governor and from 1348 as the seat of the Despotate of Morea. Captured by the Turks in 1460, it was occupied thereafter by them and the Venetians.
Today impressive ruins of Villehardouin's Castle still remain.
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.