I have no idea whether you can visit the interior of the Chateau d'If, which is on the grounds of the Chateau de Monte-Cristo, since we only had time to rush through the main chateau, the chateau d'If is a smaller building on the grounds that served as Dumas' study where he wrote. The facade is carved with the titles of his works and decorated with sculptures of some of his fictional heroes.
The real Chateau d'If is a fortress built by Francois I located on the island of If off the Marseille coast, it was later used as a prison and was unescapable. The Chateau d'If was made famous by Dumas' novel "The Count of Monte Cristo", his characters Edmond Dantes (the count) and Abbe Faria did escape the unescapable prison.
The Chateau de Monte-Cristo was the home of author Alexander Dumas who built the chateau in the 1840s after his books made him rich. Dumas is the author of classics like "The Three Musketeers", "The Count of Monte Cristo", "The Man in the Iron Mask".
The house warming party at the chateau was in 1847, he had racked up so much debt with his lavish lifestyle that he had to sell the chateau in 1849 although he was allowed to live there until 1851. The chateau really isn't that large, certainly not compared with the chateaux we would see later in the week such as Vaux-le-vicomte or Fontainebleau or Chantilly. And the decor not at all remarkable with the exception of the Moorish room which instantly set my brain whirling on how to create one of these rooms at my house!
No photos allowed on the inside of the chateau but the website below has a photo of the stunning Moorish room.
While I can understand the concept of a siesta in Spain where the midday temperatures can sizzle, it was a little surprising to see a tourist sight in France that was closed between 12:30pm and 2pm. The staff is quite serious about this at the Chateau de Monte Cristo, at 11:55am they wouldn't let us into the parking lot and when we parked nearby and walked in, they sold us a ticket but said that we MUST be out by 12:30pm. And when we entered the chateau, the chateau guard looked dubiously at his watch and reiterated that we MUST be out of the house and off the grounds by 12:30pm. We really didn't have the option of spending an extra 2 hours in Port Marly, we wanted to get on the road to Chartres and quite honestly there isn't anything else to do nearby that would fill 2 hours.
Despite what the frosty clerk at the Chateau de Monte Cristo said about the Chateau being well sign posted, we did not see one sign suggesting that the Chateau was even in Port-Marly much less any sign pointing the way. Most every historical sight or chateaux in France is signposted with brown signs at roundabouts or near where you need to turn so we had come to expect it while driving around France. If you plan on visiting, print out directions from where you think you are coming from or print a map from their website, at least it would be something to show the locals who also don't seem to know that the chateau is there.