Walls of Paris
Various Walls were built around Paris, some for protection, some used for commerce to enforce the payment of a toll on goods entering Paris
https://en.wikipedia.org/wiki/City_walls_of_Paris
Defensive Walls
https://en.wikipedia.org/wiki/City_walls_of_Paris
Walls to enforce the payment of a toll on goods entering Paris
https://en.wikipedia.org/wiki/Wall_of_the_Ferme_g%C3%A9n%C3%A9rale
Critical elements of a wall is its Gates (or Portes) where you can pass through the wall
These Portes which remain are critical locations of the city, and are often marked by Metro Stations
Although evidence of the Wall of Paris is scarce, many Structures located at the Port remain