Yes, a building is often called XY Castle -- especially if it was built on the site of a former castle or even converted from a castle -- but I don't think it would necessarily be right to call it "a castle". (Edit: ... though you might well call it a castle if it was built in the style of one, with battlements, etc.)
"Schloss" is really difficult. In British English we wouldn't say "chateau" unless it was in France, and few buildings would qualify to be called palaces. You can speak of the "country seat" of an aristocratic family. Otherwise I think one has to find an appropriate description, such as a (grand) country house.
For some houses in Britain, of course, we use the term "stately home" -- the kind of splendid country house, formerly or still owned by an aristocratic family, that is often open to visitors.