Another one of my favorite episodes. I agree, Matti, the opening scene, with the music setting the mood, is creepy. This befits the nature of Dr. Mason. I especially like the way the camera slowly pans the Mason house, revealing little clues which will come in handy later. The fact that there is no dialogue, other than the two utterances of "Rosebud", reminds me of the opening scene of "By Dawn's Early Light". The music in this episode (and most of the last two seasons) is outstanding. You get the feeling right away that this is not just a TV show, but a real movie.
One thing I never noticed until a few years ago, was that you can see the tape recorder in Columbo's raincoat while he sits by the fire with Dr. Mason. It's in the foreground, but it's not in focus, which makes it tricky. A nice touch. This reminds me of the scene where you can see Vincent Pauley's medical ID bracelet, giving away a key clue in "The Conspirators".
Dr. Mason probably takes the cake for most attempted/completed murders: (1) He murders Charlie with the dogs, (2) We can assume he probably murdered his wife, (3) He's got his hands around Joanne when Columbo walks in, and (4) He puts the dogs on Columbo, assuming they would kill him as they did Charlie. Throw in the fact that he tried to kill the dogs, as well, and you've got quite the maniacal murderer.