It was iron and the secret of iron-making that gave the Celtic folks a leg up on those who first occupied the island. The oldest iron artifact yet to be found on the island dates from around 600 B.C. Found in the territory of a tribe called the Silures, it represents the fact that my Y-DNA arrived to the south of what has become Wales. Other Celtic tribes, based on their family unit, established themselves among the hills and mountains, valleys and dales, shore and seaside.
The figure to the right shows roughly the territories that came to be occupied by the Celtic R1b1 DNA. The extreme north was settled by the Decengli. The Clwyd would be a landmark. The Venedotae and Ordovices came to occupy the northwest sections, including the all important island called Mon. The Demetae, with a lot of Irish influence took control of the southwest. The Cornovii became the guardian of the rich lands of the middle Severn valley. Each tribal group centered around its Celtic heritage. Hill-forts became the norm. Protect what you claimed, and keep it in the family.