Materials used to build castles include earth, wood and stone. Motte-and-bailey castles were made of earth and wood, but due to weaknesses in these castles, stone castles were created.
Motte-and bailey-castles were built on a motte, which is a man-made hill. The bailey, a name for a large area of land, was next to the motte and was the site of most of the castle activities. The size of these castles was limited due to wood's weakness as a building material. The wood also rotted, particularly when it rained, and was easy for enemies to burn. If too much weight was on the motte, it collapsed.
Stone castles were more durable because stone doesn't rot and is much stronger than wood. These castles were built higher than wood castles, allowing those inside to see for miles. The walls were thicker, and the castles were larger, allowing them to keep more soldiers.
Castles had several defensive features. A machicolation, or murder hole, was built above the castle's main entrance. Soldiers by the hole could pour boiling water on invaders. A keep was a tower built in the center of the castle. Soldiers could fire on attackers from the keep. The land around certain castles was referred to as a "killing field," because invaders had nowhere to hide.