Okay, since you probably associate Montmartre with a different Catholic architectural wonder, here’s the big place (the basilica).
The other large building up here is ignored by most, skirted, and otherwise overlooked. Of course, it’s designed to disappear into the landscape, which is quite a feat considering its bulk and proximity to the two churches. It is the Réservoir du Montmartre—yes, a hilltop water storage facility.
If you’re wondering, we came up streets and steps from the northeast, and came down the “front.” Didn’t see Amélie.