Mont Saint-Michel Abbey

Mont Saint-Michel Abbey (モン＝サン＝ミシェル修道院) is an abbey built on a rocky tidal island of the same name. It is one of France's most recognizable landmarks.

Index Arc
Stiyl Magnus of Necessarius mentions that Index has been to the island prior to the start of the first volume in the timeline alongside the Vatican Library, the Pataliputra ruins, Château de Compiègne, and Louvre Museum. There is apparently a grimoire or more sealed in the abbey which cannot be taken away.

Angel Fall Arc
It is mentioned again by Tsuchimikado Motoharu as being one of the few places that has strong anti-magic defenses like St. George's Cathedral, where the people inside could have escaped some of the effects of Angel Fall at its activation.