The Tour Saint-Nicolas is the southern of the two towers guarding the entrance to the Vieux Port — a hefty stone keep built between 1345 and 1376, when La Rochelle's wealth was at its medieval peak. Inside, a maze of spiral staircases and small chambers climbs to a terrace at the top, with a long view across the harbour to the Tour de la Lanterne and out to Île d'Aix.
The interior was used as a prison for several centuries; you can still see the cells and the graffiti scratched by their occupants. The structure is slightly leaning — the medieval foundations were never quite up to its weight, and it has been settling into the marsh for over six hundred years. Joint tickets cover all three towers; allow two hours for the visit.