"The moon was shining brightly and I saw clearly that no one had touched the window. Not only were the bars that protect it intact, but the blinds inside of them were drawn, as I had myself drawn them early in the evening, as I did every day, though Mademoiselle, knowing that I was tired from the heavy work I had been doing, had begged me not to trouble myself, but leave her to do it; and they were just as I had left them, fastened with an iron catch on the inside. The assassin, therefore, could not have passed either in or out that way; but neither could I get in."
I read this for my "locked room mystery" square and I really liked it. I spent most of the time coming up with, and subsequently discarding, solutions for the impossible crime, as did most of the characters!
The "hero" of the piece of Rouletabille, who is a journalist, and whom the narrator describes as a bit of a wunderkind. He is the only one who manages to figure out what was really going on with the murder and murderer.
While this is a short book, it does take some time to read. It's a translation, and was originally published in 1908, so it's not an easy read. Focus is required to keep track of the characters and the events. I didn't figure out the solution at all - I thought I had it figured out, and then it turned out I was entirely wrong about everything. Which means that it was a success!