When Castle and Beckett investigate the murder of a man with human bite marks on his body, Castle's wild theories start flying. But none are as wild as what their only witness insists happened - a Zombie attack. As the evidence pointing towards an undead assailant piles up, the team plunges into New York's Zombie subculture to find the killer and bring him in—dead, undead or alive.
Pearlmutter: Ah, our intrepid heroes have arrived. And Castle.
Castle: Ah, Perlmutter. I will treasure these special moments we’ve shared.
Beckett: It sounds like Charlie’s having a psychotic break. What if his guilt made him snap and he created a fantasy in which zombies killed David and not him?
Castle: I’m not so sure. I mean, his voice does have that authentic ring of pants-wetting terror.
Beckett: You do not believe in Charlie’s story.
Castle: I believe that he believes it.
Castle: Our killer's a zombie.
Ryan: I mean, he does look like a real zombie.
Esposito: A real zombie? I’m embarrassed for you, bro.
Beckett: Castle, call for backup!
Castle: This isn't over. This isn't over.
Beckett: What did you find at the crime scene canvass?
Esposito: Nada. Only in New York can some guy dress like a friggin' zombie and walk down the street unnoticed.
Ryan: Castle, do you really believe in all this zombie stuff? Because I would swear on my nana's grave that Kyle Jennings was dead.
Castle: No. You know what I do believe in? Driving Beckett crazy.
Alexis has tentatively decided to go to Columbia, a school in New York City.
Castle reveals another longstanding secret in this episode... to Ryan. His crackpot theories and wild speculations are often motivated by one thing: to drive Beckett crazy.
Of the many undead references throughout the episode.
World War Z is a novel by Max Brooks (with a film adaptation in production at the time that the episode was produced).
Beckett, if she could be any supernatural creature, would choose to be Van Helsing. This is a character in Bram Stoker's Dracula. In the original book and most of its adaptations, Helsing himself isn't supernatural at all.
Castle comments that being a "monster slayer" is very fitting for Beckett. This is yet another reference to Buffy the Vampire Slayer, where Nathan Fillion played a villain.
While the use of mind control drugs is a popular trope in television and urban legend, scopolamine cannot force a person to perform acts against their own morals. It is for this reason that most states do not consider being under the influence of a drug, especially one taken voluntarily, to be a defense in the case of the commission of a crime.
Castle once attempted to write a comedy about the Civil War. After significant research, he realized that there was nothing funny about it.
Castle and Alexis have been playing laser tag since she was five. The entire time, they've been in a game to one thousand. Since age ten, Castle has been unable to surprise his daughter.
When Castle first arrives at the crime scene, he is not carrying his traditional cup of coffee for Beckett. More importantly, she's brought her own cup, which she is discussing with Esposito when Castle arrives.
Castle responds to Perlmutter's sarcastic greeting by saying, "Ah, Perlmutter, I will treasure these special moments we've shared." In addition to implying that he will soon be leaving, this is very close to how he greets his exgirlfriend, Kyra's mother, in A Rose for Everafter. She had always been hostile and disapproving of Castle.
Beckett has only made moves towards a romance with Castle when confronted with the possibility of him leaving altogether. While this episode marks a major improvement in their relationship, it is no exception. For his part, Castle only commits to staying once Beckett obliquely confesses that she is dealing with her trauma and will soon be willing to have a relationship.
Castle editor Marta Evry was an uncredited extra who was the zombie woman with bluish hair in the zombie walk.
The zombies portrayed in this episode are similar in appearance to the Reavers from Firefly, which starred Nathan Fillion. 
"The killer's a zombie." and its attendant high five are part of Beckett's evidence in "Still" that Castle isn't, in his words, "almost always right".
Another similarity to Firefly is the realization that a supposedly dead body is actually still alive. In this case, Kyle Jennings wakes up in the morgue just before Perlmutter is about to perform an autopsy on him, similar to how Tracey Smith woke up in the med-bay of Serenity just before Simon performed an autopsy on him in the Firefly episode The Message.
When Castle gave Alexis the lazer tag outfit, he said "suit up", which is a popular phrase by Barney Stinson in "How I met your mother", a frequent player of lazer tag.