What is a castle?

Land owners and lords built sturdy castles to defend themself.
The castle was designed to withstand an attack and to mount a defence.
At first castles were made out of wood.
They stopped making castle from wood because it was too easy to burn.
They started to build castles from bricks and stones.
The castles were surrounded by thick walls.
Inside the thick walls there were many buildings.
The common people use to rush to get inside the castle if there were signs of trouble.