The Art of Castle Building. Dibnah travels across the north coast of Wales, stopping at Harlech, Caernarfon and Beaumaris, to investigate one of the greatest feats of royal engineering -the chain of castles spanning the area. In the second of this week's programmes he reveals the building techniques and revolutionary defensive features that helped Edward I subdue the rebellious Welsh, as well as charting the achievements of the king's architect, Master James of St Georges, a Frenchman who revolutionised English castle design.