We tend to glamourise castles,as having been fun places to live.Mostly,pre-16th century,that wasn't the case.When William and his Norman hordes invaded and took over England,they threw up motte and bailey castles everywhere they went.Firstly,because they were relatively quick to construct,and secondly,because they had to,as they(The Normans) weren't exactly welcome.Castles,in Britain,started as "castellum" or a fortified place.A secure holding in times of strife.As times changed so did requirements,and thus castles changed.Some were fortified all the more,some abandoned altogether,some were remodelled into mansions.More often than not as they were abandoned ,stone robbing took place,leaving only the earthworks,in the form of mounds and ditches,for future generations to ponder over.A large number of castles,in England and Wales,exist now ,only in name,with modern farming methods having removed them from extant existance.Lots are visible now only from aerial photography.And yet,they all helped to fashion the land as we see it today.
see http://en.wikipedia.org/wiki/List_of_castles_in_England