There are a lot of different ways to describe "Castle" to someone who's never seen the show, but I don't think "subtle" would ever be one of them. Still, I don't remember an episode of the show that was quite as obvious in both its cross-promotion and its underlying message as "Heroes and Villains", the episode that aired Monday night.
The basic story featured a so-called "real life" superhero committing a murder, and Castle and Beckett trying to track down the killer, who turned out to be an impostor hero after many twists and turns. It was typical "Castle" fare and enjoyable enough if you didn't mind that once again the killer was the first suspect brought in, but they didn't figure that out until the end (that happens a LOT on this show).
But what wasn't typical was the overwhelming way in which product placement and integration became part of the plot. For those who aren't aware, Marvel Comics (a Disney-owned company, just like ABC, which airs "Castle") is turning one of Richard Castle's in-universe novels into a graphic novel. As you can see from the picture above, when Castle and Beckett had to visit a comic store as part of their investigation, the graphic novel was prominently featured, in a scene that came off as an in-episode commercial. All that was missing was information on how to order the book from Marvel.com. On top of that, the vast majority of comic book characters referenced by the characters in the show were Marvel characters. Spider-Man, Daredevil, Black Panther, Iron Man... the list went on and on. There was one reference to Bruce Wayne (a DC character) and one reference to "Sin City" (a Dark Horse book), but otherwise it was a 42-minute "Make mine Marvel!" promotion.
And yet somehow, that wasn't even the least subtle aspect of the episode. In the final scene, the writers of the show whipped out the metaphor sledgehammer and smashed the audience over the head with the parallels between Castle and Beckett and the cop and writer who were at the center of the superhero mystery. The mere existence of the parallels was bad enough, but the fact that the characters pointed them out in the show itself. It was practically an insult to a viewership that's been waiting for any kind of Castle/Beckett resolution for 3+ seasons now. And it's the kind of cheap chicanery I thought the writers of "Castle" were above.