An allusion that appears in the beginning of chapter VIII?

1) The Château d'If is a fortress (later a prison) standing on the island of If, which is located in the Mediterranean Sea.

"The Chateau d'If?" cried he, "what are we going there for?" The gendarme smiled.

"I am not going there to be imprisoned," said Dantes; "it is only used for political prisoners. I have committed no crime. Are there any magistrates or judges at the Chateau d'If?"