The fortress and fortifications were built on the orders of Cardinal Richelieu in the 1620s during the war with the Huguenots of La Rochelle. In this article you can read who are they.
The walls that can be seen in the town were built later, in 1670 - 1702. The first fortifications were partially dismantled after the end of the siege of La Rochelle, due to the fact that they represented a threat to royal power.
Getting there: a large parking N 46 12.312 W 1 21.751. From La Rochelle you can take the bus 3/3E, find here a link to the current schedule.