Where does the last name CASTLE come from and what does it mean?

Question:

Where does the last name CASTLE come from and what does it mean?

I havent found anybody who has been able to answer this,Please somebody if you know tell me i would love to find out.

Best Answer - Chosen by Asker: Google helps - here's a theory:
"Originally, the name Castle probably meant a person who worked at a castle, or perhaps a governor or constable of a castle."