Where: Le Mont-Saint-Michel, France

Start of construction: 10th Century

End of construction: 15th Century

Most important renovation: 21st Century

Accessible: yes

Important fact:

Not a castle, but an abbey and the third most visited monument in France

(3.5 million visitors per year).

https://www.ot-montsaintmichel.com/index.htm?lang=en

​