The castle, which founded in 1249 by the Frankish ruler of the principality of Achaia, William II Villehardouin, is at the original core of Mystras and occupies the peak of a hill at the foot of Mount Taygetos. It was constructed in order to secure the plain from the Slavs of Taygetos and reinforce the Latin domination of the region.  The Frankish castle was ceded to Michael VIII Palaiologos in 1259.
The castle of Mystras, perched at the top of a hill that has precipitous slopes except on the east, is part of a system of fortifications that utilize its naturally defensible location. There are also two parallel zones of walls below, at the base of the hill, which enclose the settlement. The castle itself also has two fortified defensive zones. The circuit wall has only one tower, above the ascent, and a single entrance. The keep occupies the height of the hill.
The two parallel walls farther down the hill were built at the same time as part of the initial fortification program, dividing the settlement into Ano (Upper) and Kato (Lower) Chora. The enclosure wall of Ano Chora has two gates, the Nauplion Gate on the exterior wall, protected by two towers and outworks, and the Monemvasia Gate in the section of the wall separating Ano from Kato Chora. Two important monasteries stand at the two ends of the wall of Kato Chora, the Church of the Hodegetria of the Brontochion Monastery and the Peribleptos Monastery, which, with their imposing fortified towers, participated in the defensive function of the walls.