Windsor Castle was originally built by William the Conquerer to protect Norman interests around London. It consisted of a motte (mound on which the actual castle stands) surrounded by a bailey (wall providing a defence). It is still a Royal Residence, being used for hosting state visits, and as a weekend retreat for the Royal family. This now forms the Middle Ward of the 'modern' castle. Upper and Lower Wards were added at various stages. A comprehensive history of the castle can be found on Wikipedia .