In 1296AD King Edward of England built Beaumaris Castle, some 15 miles north of Caernarfon Castle, as another of his "ring of stone" to control the Welsh peoples. This castle was never finished but is a perfect example of the "concentric theory". It took so long to build, they ran out of money! The King was taxing everyone to the hilt to get enough money for his castle building program.
Castles in Wales have survived very well, as life was hard and unlike many English towns, things were not knocked down every few years to build something else. The actual town of Beaumaris is on a large island, called Anglesey, (Sir Fon in Welsh.) The only way to get to it was across dangerous sand flats two miles wide. Many people, horses, and carriages sank without a trace in the mud. Today you can cross the ancient suspension bridge at Menia Straights, itself nearly 150 years old.