A photo showing the entrances to the cathedral sandbagged for protection.

https://www.abc.net.au/news/2019-04-16/notre-dame-cathedral,-world-war-i-1/11018736

And another view:

WWI. Protection of monuments during the war. Sandbags protecting the portals of Notre-Dame de Paris in 1918. [1280×937] from HistoryPorn

Blair Tarr is the Museum Curator of the Kansas State Historical Society. He oversees the three-dimensional collections of the Society, but has special interests in the Civil War, Wichita-made Valentine diners, and Leavenworth's Abernathy Furniture. In the last few years he has also done a lot of cramming on The Great War. He is a past president of the Kansas Museums Association and the Civil War Round Tables of both Kansas City and Eastern Kansas. He is currently a board member of the Heritage League of Greater Kansas City.