New ways to make castle defences stronger were found.
- The stone-built curtain walls round the bailey became thicker, stronger and higher.
- Towers were added to the curtain wall. These projected (stuck out) from the wall so soldiers had a better view of the surrounding area and could fire their bows more accurately.
- To protect the entrance (often the weakest point of the castle) the gatehouse was built. At first it was a tower with a simple archway in the curtain wall. Later on twin towers, usually D-shaped, were built on each side of the entrance. They usually projected forward to guard the gate and had arrowloops.
- Sometimes these towers were made deeper so that there was a passageway through the gatehouse. In this would be gates, portcullises and murder holes.
- The way into the castle was usually by a bridge or drawbridge across the ditch or the moat. This was a dangerous position for attackers who could be shot from the gatehouse.
The concentric castle had more than one line of defence: an outer wall with additional towers surrounded the inner stronghold. The inner wall was higher, to allow archers to shoot arrows over the heads of soldiers stationed on the outer wall. Sometimes, moats were added to make the castle even stronger.
Caerphilly Castle was one of the first planned concentric castles. It was built by Gilbert de Clare III, a powerful Anglo-Norman lord, and begun in 1268.