"Fidelis ad mortem" is Latin for "faithful unto death", and is the motto of the NYPD.
When Castle tells Beckett about what he discovered in Los Angeles, and she asks why he chose to forget, he tells her he was trying to protect her, a reason he says she knows "all too well." He is referring to their argument in "Always (2012)", when he reveals his complicity in the deal Mr. Smith made with Bracken to leave her alone as long as she dropped the investigation into her mother's murder.
Wings Hauser, who guest stars as 'Lucky' Jack Flanagan, played the father of Darby Stanchfield (Castle's ex-wife Meredith) in a season one episode of The Mentalist (2008).