Image courtesy of http://www.castlewales.com
The Concentric castle was developed in the 12th and 13th Centuries and offered the best protection against attack. The main feature of the concentric castle is its walls. An inner wall built of thick stone with turrets positioned at intervals is then surrounded by an equally thick but lower stone wall. The walls are built at different levels so that archers on the inner walls can fire over the archers on the outer walls.
The space between the two walls was known as the ‘death hole’ because being trapped within the walls would almost certainly result in death for the attacker. The entire castle was then often surrounded with a moat and entry would be across a drawbridge.