A castle is a large stone structure used in midieval times to protect against an oncoming force. Most lords and knights had one. Although castles were built to accomodate all of the people of the land in case of an invasion, people seldom lived in them, as they were cold, damp, and dreary.
In this day and age, though, it is possible to build a castle that is quite comfy to live in thanks to modern tecnology.
Most people who live in castles also have a collection of swords.