Gorgeous newly renovated apartment in an amazing location!
Marie's apartment is right on the street so you are certainly in the thick of it, but we never felt that security was an issue. There is a single step just inside the entry to the flat.
10 minutes walk to Sacre Coeur and 5 minutes to the metro, with heaps of surrounding amenity.
Like others have mentioned, there is a school close-by and a staircase above the apartment, but the noise didn't bother us. I suspect a lot of accomodation in the heart of Paris would have similar noise levels.
The flat is beautifully decorated with a comfy double bed, great shower, spotless bathoom, and large kitchen. There is not a lot of natural light, with only frosted windows at the front, but don't worry you will be too busy exploring Paris for this to be an issue!