<prompt> Translate the following text to EnglishWakati watu wa Celtic walipovuka Pyrenees katika milenia ya kwanza KK, walioa na idadi ya watu wa Iberia na kujenga safu ya jamii za ngome za kilima au citadels, mfano bora ni katika Breteiros.  <response>: When Celtic peoples crossed the Pyrenees in the first millennium b.c., they intermarried with the existing Iberian population and built a series of hilltop fort communities or citanias, the finest example of which is at Briteiros see page 73. 