The description for this category on wikipedia states;

The subcategories attempt to list all county houses, stately homes, manors, country retreats and estates, mansions, and houses in England by county—anything of historical architectural note that was used as a residence by a noble family or persons of esteem in history. This includes castles, palaces, old abbeys, priories that may at one time have been used as a noble residence, including those that now lie within urban areas such as London but retain the appearance of a country house in style, and have extensive gardens.