The Patras Castle was built around the mid-6th century above the ruins of the ancient acropolis, on a low outlying hill of the Panachaiko Mountain and ca. 800 m from the sea. The castle covers 22,725 m² and consists of a triangular outer wall, strengthened by towers and gates and further protected originally by a moat, and an inner compound on the northeastern corner, also protected by a moat.
The castle was built by Byzantine emperor Justinian I after the catastrophic earthquake of 551, re-using building material from pre-Christian structures. One of these spolia, the torso and head of a marble Roman statue, became part of the city's folklore, a sort of genius loci. It is known as the "Patrinella", a maiden who is supposed to have been transformed into a man during Ottoman times, guards the city against disease and weeps whenever a prominent citizen of Patras dies.