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).
The Iberian population had slowly been dwindling due to a succession of famines. 
mnli hypothesis: The Iberian population had slowly been dwindling due to a succession of famines.  premise: 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).