where did it get the name from?
It is derived from the Latin word "castellum", which is a diminutive of the word "castrum", meaning "fortified place".
Castle
The word "castle" is derived from the Latin word "castellum", which is a diminutive of the word "castrum", meaning "fortified place". 
who brought the idea of castles to england Discussions have typically attributed the rise of the castle to a reaction to attacks by Magyars, Muslims, and Vikings and a need for private defence. The breakdown of the Carolingian Empire led to the privatisation of government, and local lords assumed responsibility for the economy and justice.
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Castle [SEP] Etymology
The word "castle" is derived from the Latin word "castellum", which is a diminutive of the word "castrum", meaning "fortified place". The Old English "castel", Old French "castel" or "chastel", French "château", Spanish "castillo", Portuguese "castelo", Italian "castello", and a number of words in other languages also derive from "castellum". The word "castle" was introduced into English shortly before the Norman Conquest to denote this type of building, which was then new to England.