"Murder, mystery, the macabre." This is the way Michael Connelly began his introduction of Castle at the Poe's Pen Award dinner. Those are also the same words that Gina (Castle's publisher and second ex-wife) used to introduce Castle at the launch party for the last Derek Storm novel in the pilot "Flowers for Your Grave".
Season 6, episode 5 with the "time traveler" alluded to Senator Beckett.
On the Poe's Pen awards posters on the stage to either side of the speakers (Michael Connelly and Nathan Fillion), the diagonal stroke on the "N" is a pen just like in the Castle title/opening credit screens.
Nathan Fillion and Lance Reddick have previously performed together in Destiny (2014) as the Vanguard trainers Cayde-6 and Commander Zavala respectively.
Interestingly the awards are the Poe's Pen awards. Castle had changed his name from Richard Alexander Rogers (birth name) to Richard Edgar Castle, after the writer Edgar Allen Poe, as first mentioned in S3E2 "He's Dead, She's Dead"
Andrew W. Marlowe: The Castle (2009) series creator appears at Castle's Poe's Pen award ceremony seated with Michael Connelly, along with other members of the show's writing staff.