Except for Don Sebastian de Morra, about whom nothing is known, the remainder of this story is broadly based on historical characters and events. However, I’ve taken a few liberties with events and timing for the sake of dramatic interest, principally:
Cinq-Mars was not an orphan. His father Antoine died when he was young and Richelieu was closely associated with the family.
Cinq-Mars was not romantically involved with Chevreuse.
Chevreuse was from the House of Rohan and was not born a farmer’s daughter.
The paternity of Louis XIII’s children is a subject of historical debate. However, Cardinal Mazarin is one of the many people touted as the possible father.
Richelieu gave the Palais-Cardinal to Louis in 1639, and it was known as the Palais-Royal thereafter.
In the Oxford edition of The Three Musketeers, David Coward states that Richelieu was in fact an admirer of Machiavelli, and therefore did not think him naïve.
Richelieu did not stay in Paris during the siege of Corbie, but in fact took the town on 14th November 1636.
The siege of Arras took place before Richelieu’s vetoing of Cinq-Mars’ proposed marriage to Marie de Gonzague.
Spain was far more deeply in debt than the book implies and had largely ceased to be a threat by 1640. The Spanish were finally defeated by the French in pitched battle at Rocroi in 1643, ending a century of dominance. Cardinal Mazarin was to continue Richelieu’s work centralising the state, under Louis XIV – the Sun King and builder of Versailles.
Many books have been written on this period of history. Two I found particularly useful were Cardinal Richelieu: The Making of France by Anthony Levi and Louis XIII by A. Lloyd Moote.