The hardest people to subjugate are tight-knit clans and tribes. People who know each other well, have a lot in common, and, ideally, share a geographical area. The celts successfully held back the might of the Roman Empire, not because they were better organized, or had better technology. They held them back because they had something to fight for, and that made them ferocious. Hadrian built a wall in England to stop their counter-attacks. The Empire had essentially admitted defeat and realized it could never conquer past the wall.