If you like ramparts (who doesn't?), Provins is a really good destination for a little side trip from Paris. It's 77 kms south-west from the capital, in the département of Seine-et-Marne (77).
Back in the good ole' days of the Middle Ages (Moyen-Age, in French), Provins was pretty high in the city charts, being 3rd after Paris and Rouen, thanks to a giant trading fair it used to host as part of the Champagne region (Provins even had its own money). Provins was well known for its main industry, linen. The old city is riddled with underground passages, remnants of quarries that were extracting Fuller's earth (in French, terre à foulon), used in the process of degreasing wool. The "souterrains" are featured in Umberto Ecco's novel, "Foulcault's Pendulum".
The fortifications are what makes Provins special. They were built from 1226 to 1314. The "Tour César" (shown above) is the only octogonal dungeon with a square base known to man (and woman). It is rumored its earliest version was built by order of Julius Cesar, hence the name.