Overview: Fred Dibnah's Building of Britain continues with the famous steeplejack investigating one of the greatest feats of royal engineering - the constuction of the great chain of castles on the coast of north Wales. Travelling to Harlech, Caernarfon and Beaumaris, Fred reveals the building techniques and revolutionary defensive features that helped Edward I subdue the rebellious Welsh. Fred also charts the achievements of the king's architect Master James of St Georges, a Frenchman who revolutionised castle design in England.