Concentric Castles

Example of a Concentric Castle

     A concentric castle is literally a castle within a castle. These castles were designed by architect Master James of St. George for King Edward I in Wales specifically to withstand sieges and destroy the attackers. These castles were surrounded entirely by moats and had several gatehouses. The interior hosted fresh running water, plumbing, and larger windows for the inhabitants. The disadvantages of such a large castle rested heavily on the upkeep of it. A vast amount of skilled workers were required just to keep a single concentric castle in good condition after being built.

Beaumaris in Beaumaris, Wales

     Concentric castles were built with hard chalk, flint, limestone, and sandstone over a fresh water well or a spring. Like any castle, a heavy siege of ballistas and trebuchets could bring down the walls which made the curtain (outer) wall effective against the attackers. Even when attackers managed to knock down part of the outer wall from a distance, they still had the moat to contend with first. The moat could be filled with any number of things ranging from sewer water with wooden spikes to flammable oil or tar. If the attackers managed to breach the moat and pass through the outer wall, they would be inside the outer bailey with defending archers above them. A death trap for the attackers before they're even able to breach the inner wall.
     All of the towers were rounded, as you see in the first picture. This prevented blind spots and mining underneath corners to collapse entire walls. With a wider range of view, defenders could attack from different directions and there was no corner for the attackers to hide around which made these round towers far more effective. The inner walls were built higher than the outer walls so that the defenders could see their attackers on the other side of the moat.
     If you would like to learn more, here are some follow-up sites on concentric castles.
http://en.wikipedia.org/wiki/Beaumaris_Castle
http://www.ancientfortresses.org/concentric-castles.htm