Two monuments in Paris are so prominent that they’re hard to miss. One is the Eiffel Tower, of course, the all-iron tour de force of engineering, standing by the Seine amid the city’s spacious and supremely elegant West End. Then to the north, atop Montmartre, there is the Sacré-Cœur: a tall, immaculately white Catholic basilica that looks like a digitized pre-Raphaelite set from “Lord of the Rings.” What most visitors—and in fact most Parisians— don’t realize…
