There is a secret passageway underneath the elevator. You need to figure out how to get into it, and then once you do, go through the passageway to get to the tower. Along the way, there are a series...
If you go to www.google.com then type in "walkthrough for treasure in the royal tower" it will give you the step-by-step process of what you need to do.
Mostly. It is loosley based off the age-old tale of Marie Antoinette and her secret tower. I'm not sure if the tower part is true or if Windsor Castle is even real but it is still loosley based of...