Rating the Ratings: It was a somewhat weird season for Castle's timeslot; The Blacklist on NBC was far more formidable than anything Castle had ever faced on NBC, but Hostages and Intelligence were far weaker than anything Castle had ever faced on CBS. These changes almost cancelled each other out in pure ratings terms, but The Blacklist may have crossed over with Castle's audience even more than previous NBC occupants. Castle has been additionally impressive in recent seasons because of the collapse of lead-in Dancing with the Stars, but it doesn't get that extra credit this year as the reality franchise steadied the ship.
Usually I'd consider a B+ for down mid-single digits, especially when it's the show's second-biggest season in A18-49+. But I have to nick Castle a bit because it finished the season a lot worse than it started. It was actually +5% in raw numbers as of the end of the fall, but almost all points in the second half of the season were down double digits. It didn't seem to benefit from the return of DWTS in the spring, even though DWTS itself had a very good season. Grade: B.
Here's the now updated War of 18-49 post for Castle.