One of the most famous castles is the Tower of London. The Tower protected the Normans from the rulers of England.
The Normans had trouble maintaining control, so they built many castles. These castles served as a large fortified homes for the nobles, and were large enough to provide protection for the peasants surrounding the castle.
The first castles were made from wood and earth, then they added stone for strength. Norman castles were normally built in Motte-and-Bailey style.
A motte is a man made stack of dirt with a flat top. This was usually made for self-defense and sometimes there were buildings on it.
The keep of a castle is the safest place from the attacker and the strongest. It was usually built on top of a motte, and was where the ruler lived. The bailey is the fortress surrounding the keep. And at the outer wall of the castle would be a "curtain wall". The curtain wall was surrounding the bailey, they were high and thick, so it would be hard to climb over and penetrate.
There was also usually a moat around the castle. A moat is a deep wide ditch surrounding the castle sometimes filled with water or could be dry, but it was to keep ladders from reaching the castle and to prevent the enemies to dig under the wall. There was also a drawbridge to cross the moat and could be lifted during a battle.