Kayseri was first mentioned in the 4th century A.D., known by the name of Ceasarea. Its walls encompassed a vast area, so, to improve its defensibility, the Byzantine emperor Justinian I contracted these city walls and had the castle built in a corner of the city walls in the 2nd quarter of the 6th century. The word "kale" can mean castle (as in the text before) but also "fortress".