Comparing two restored icons
Carcassonne vs Mont-Saint-Michel
Both UNESCO-listed, both rebuilt into the silhouette everyone recognises — but one depends on the tide and the other doesn't.
Check dates on Viator ↗A walled town versus a tidal island
Carcassonne is a hilltop town you can walk into at any hour, any day, by road or rail. Mont-Saint-Michel is an abbey on a rocky tidal island about a kilometre off the Normandy coast, reached by a bridge across the flats that opened in 2014 — and the bay's tides, with a range of up to about 14 metres, set a rhythm Carcassonne's visitors never have to think about.
Two different building stories
Carcassonne's walls owe much of their present look to Eugène Viollet-le-Duc's restoration from 1853. Mont-Saint-Michel's ramparts are largely Hundred Years' War work, with major fortifications credited to the reign of Charles VI. Carcassonne's restoration is the more argued-over of the two, mostly because of its roofs.
What each one actually is
Carcassonne is a fortified town — streets, shops, a free basilica, and one ticketed castle-and-ramparts combination. Mont-Saint-Michel is an abbey crowning a small village on its rock; there isn't a second town's worth of free wandering once you're past the gate.
Which to see first
If you're building a route through southern and northwestern France on one trip, there's no real overlap to worry about — they sit on opposite sides of the country and reward completely different kinds of visit. Neither is a substitute for the other.