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As the name suggests, concentric castles were based on a ring-within-a-ring structure.

Sometimes known as a multiple castle, the buildings were constructed to present as daunting a challenge to attackers as possible.

Sometimes on a rise, and sometimes bounded by a moat, concentric castles were far larger than the earlier motte and bailey designs - and could never be built on man-made mounds.

There would be a central enclosure of stone walls and towers, then one or more outer walls, also with towers. The walls would be castellated and usually featured arrow slits and outlets for boiling oil or other missiles.

The height of the walls would increase towards the middle, enabling inner defenders - such as archers - to shoot over the defenders of the outer walls.

Great examples of concentric castles in Wales are Caerphilly and Beaumaris.