The Fort can be visited
Fort Saint-Nicolas

At the harbor entrance of Marseille in the Provence-Alpes-Côte d’Azur region of France is Fort Saint-Nicolas. It was once built as a twin fortress to Fort Saint-Jean and served to control the city.
History+
Following civil unrest in 1630–31 and the Fronde from 1648–1652, King Louis XIV decided to erect two large fortresses at the harbor entrance of Marseille to oversee the city's population. Fort Saint-Nicolas was built simultaneously with the opposing Fort Saint-Jean. Louis XIV remarked that the inhabitants of Marseille had a fondness for fortresses, and he therefore wanted his own at the entrance to the great port. In fact, the cannons of the two new forts were pointed towards the city, not outwards towards the sea.
Source: Wikipedia · Licence: CC BY-SA 4.0 · Image: Hervé Rebours
This description draws on the facts documented in the linked Wikipedia article; Wikipedia content is licensed under CC BY-SA 4.0. Wording and editing: Mobile History.
Location: 43.2910, 5.3625