I think this is the first Alleyn mystery where I genuinely felt for the victim, which helped greatly in my enjoyment of the story. It’s the first one where you spend part of the book following the victim closely, too, and where Alleyn has personal feelings on the matter, both of which I think are relevant. I know that the trope of the personally involved detective can be exasperating — and Alleyn even refers to it, in one of those unsubtle bits of meta — but at least it’s another way for the reader to engage with the case.
The actual puzzle aspect of the story is more or less as usual: a character you wouldn’t normally suspect ends up in it up to the neck, where the guy who looks like a sure thing is actually innocent. Still, the reasoning does make sense, all the timings match up, etc, so it makes perfect sense, which Ngaio Marsh is admittedly good at (apart from the weird mix of opportunism and premeditation in the crime in the first Alleyn book).
As for Alleyn’s personal life, well. I still can’t help but feel he’s a cut rate Wimsey. His relationship with Troy has some similar ups and downs to Wimsey’s with Harriet, but we don’t get to see as much interaction, as much of the push-and-pull they feel, and so it feels less compelling. I know I’m biased as a major fan of Sayers already, but I can’t help the feeling.