A few thoughts: Seeing Boris Karloff share the screen with Boris Karloff is a rare treat, and while this movie could have easily gotten by on that gimmick alone, it actually has a pretty cool story to tell. Karloff plays a pair of twins, the younger of which is prophecized to kill the older in a particular room in their castle. One brother is evil and the other is good, but they switch it around so the good is meant to be the killer. While the movie could have taken this and went in a more outrageous route, I was impressed by how they handled it, and Karloff does a fantastic job as both of these characters. As I said before they use some trickery to have Karloff appear twice in single shots, something that genuinely surprised me: I expected strictly over-the-shoulder shots with body doubles. As could be expected of a '30s Karloff mystery/thriller/horror(?) this movie isn't terribly long, and at just over an hour long they managed to tell a complete story. This may not be the best of his career, but it's still really solid and has a cool castle setting.
Who would I recommend it to? Anyone who's already seen all the essential '30s horror classics and wants to branch out a bit.