The defensive parts of early Norman and Saxon castles were: -
The primary defences of a ringwork type of castle was the ditch, bank and palisade. A ditch was dug around the edge of the bailey and the earth taken out was piled up inside to form the bank. Where possible, the ditch was allowed to fill with water from a nearby river or stream to provide extra defence. On top of the bank a palisade of wooden planks or logs was constructed to add extra height. A wall walk was usually built behind the palisade to allow the defenders to see over the top and fire missiles down on attackers below who were attempting the climb the bank. The palisade continued all the way around the edge of the bailey and its only real threat was from fire. This threat eventually lead to the introduction of stone for building material.
The area within the palisade is called a bailey and are mostly circular in shape. If attackers did manage to cross the ditch and get over the palisade, the people inside the castle needed a second line of defence. The keep was designed as this second line of defence. Made usually of wood, the keep needed to be large enough to hold the baron's family and household. Space could also be required to hold soldiers and local villagers at times of attack.
Types of keep
In some early castles the strongest part of the construction could have been its gatehouse in which case this building would have been the keep
A keep could have been made in the shape of a tower with a couple of floors or could just have been a wooden hall. Most of the early keeps were constructed from wood and were always under threat from fire. A keep built from stone was more secure but was expensive to build and only the richest barons could afford a stone keep.
Hall keeps were very common and most Norman barons and Saxon thegns depended on the protection they gave. These hall keeps needed to be large enough to house not only the baron's family, but his supporters and their animals. Inside, the hall keeps looked like large barns with huge posts supporting the roof.
A large fire was situated at the centre of the hall away from any wood that could catch alight. The smoke would rise into the rafters and exit through a small hole in the roof above or through a gap at the end of the hall.