The four castles have stood for seven centuries as a statement of imposed political union as well as the imperial ambitions of the King of England, Edward I.
Caernafon was designed to be a fortress as well as a palace and administrative centre. The castle and walls surrounding the town are the World Heritage site. The same is for the magnificent castle ensemble of Conwy, which is the finest example of a town wall in Britain. Beaumaris Castle (Beautiful Marsh) on Anglesey is technically the most perfect of the medieval castles in Britain with a double ring of curtain walling which is without parallel. The dramatic site of the Harlech Castle on a rocky out crop above Tremadoc Bay diverts attention from the most ingenious close set defensive fortifications, which are outstanding. The castles of Beaumaris and Harlech were built by the greatest engineer of the day, James of St. George. All the buildings are extremely well preserved.
In the former medieval principality of Gwynedd, in northern Wales, the castles of Beaumaris and Harlech, and the fortified town complexes of Caernafon and Conwy all bear witness to the works of colonisation and defence carried out throughout the reign of Edward I. The combined significance of the sites are the examples of medieval military town planning and outstanding examples of military architecture of the time.