If you're going to Windsor Castle, you should dress like a princess. (Not like a Disney princess.) Or like an actual princess (Princess Beatrice/Eugenie impersonators get a frosty reception). What I mean is, you should look like a commoner princess, (like Kate) elegant, dignified, strong, and a little bit naughty. See Emma and her non-frothy, non-pink Ralph Lauren Collection skirt and shirt. The best bit? Her pretty up do and slick of red lippy, giving the whole outfit a knowing finish.