My Parisian spouse surprised me by ending our long day of tourism from Paris by automobile. I'd known we were going to Giverny, where we'd been once before, but this time to see Monet's house, gardens and lily pond. We then had lunch in Vernon and looked around there. This was a surprise -- a visit to a rare troglodyte church. He'd phoned La Mairie d'Haute-Isle in Val-Oise (all three places we visited were on the river Seine, through the Vexin and to where Normandy begins) and had arranged to borrow the key. The church is completely carved out of rock, and was done in the 17th c. It was wonderful, and I have a long clip up of it on YouTube.
A lot of things in France are locked, but if you ask in advance or ask around when you get there (that's a risk -- they might be on vacation or something) you can often go to a bar, a neighbor, or as in this case, the village Hall, to borrow the key -- sometimes you will be accompanied, perhaps by the local historian. A tip is in order if this is the case.
YouTube: SLOBOMOTION, CUTECATFAITH