The Ceretani (or Cerretani) were ancient people of possibly Iberian origin, who occupied the current Cerdanya (from which derives the name) and overall the valley Segre. Their neighbours from the east were Ausetani and from the south Ilergetes. Their main town was Julia libyca (Llívia).
Their capital was located most probably on the place of modern Castellciutat.
References
Ceretani Wikipedia(Text) CC BY-SA