His new castle was built in the style of a concentric castle (which was quite cutting-edge for the time), albeit on a very modest scale.

After a second phase of building was undertaken in the 13th century, Krak des Chevaliers became a concentric castle.

Beaumaris - arguably the most perfect concentric castle in the world - began to take shape late in 1295.

When concentric castles weren't possible, the Normans built linear structures.

They can also be nested one inside the other, as in a concentric castle, giving an outer ward and inner ward.

Rhuddlan was planned as a concentric castle.

Later examples of defence in depth might be European hill forts and the development of concentric castles.

The relationship of the concentric castle to other forms of fortification is complex.

By contrast, Château Pèlerin was not a concentric castle, as the side facing the sea did not require defensive walls.

While a concentric castle has double walls and towers on all sides, the defences need not be uniform in all directions.