Palais Royal Paris
Named the Palais Royal, this stately chateau was little occupied by royality through the centuries. But Cardinal Richelieu did live in the Palais Royal. So the name Palais Royal is a bit of a misnomer. Today it houses France’s Constitutional Court and the Culture Ministry.
The Palais Royal is where Robert Langdon follows the Arago Rose Line or Paris Meridian. Langdon used the Meridian to find the Holy Grail. The Paris Meridian was displaced by Greenwich as the world’s 0 longitude line. Not sure if you will be able to find the path to the Holy Grail at the Palais Royal but you will, if you look long enough find some of the medallions of the Arago Rose Line. I found some near the gardens.
The Palais Royal is in the 1st district of Paris just next to the Comédie-Française and not far from the Louvre Museum and Jardins des Tuileries, metro stop Palais-Royal/Musée du Louvre.