During the centuries many French prisoners were sent there, from the vilest criminal, to the enemies of the state and religious offenders.
The boat leaving the Old Port and heading toward Chateu d If.
Château d'If was successful at giving potential invaders second thoughts and it was never actually tested in combat.
The process of getting there and back was slightly unpleasant.
You can purchase your tickets from any of the many pushy salespeople in the area.