Paris was still a walled city going into the twentieth century - it was enclosed by seven successive walls. Ironically, virtually nothing remains of the newer walls, while numerous vestiges of the walls of Phillipe Auguste (built 1190-1213) can be found all around the center of the city. The few images here (around a map of the old walls superimposed, very approximately, on a modern map) are only some of what remains (though many of the others are hidden in courtyards, restaurant cellars, etc.)
For those who read French, this site is about as complete a look at Paris under Phillipe Auguste as one could wish, and includes two clickable maps, with images of what must be every single vestige of these walls.
French Wikipedia article on the wallsAgain in French, this article provides a good history of the wall.
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