Credit: Photo: Nathan Fillion as Captain Hammer in Dr. Horrible's Sing-Along Blog in 2008

Castle’s fourth season is set to get super-powered! At the 2011 TCA Summer Press Tour, ABC President Paul Lee dropped a tantalizing spoiler about the newest season: a superhero will be making an appearance in an upcoming episode.

“We’ve got a really cool episode with a Marvel superhero coming up,” Lee said, later laughing that creator Andrew Marlowe was “gonna kill me” for spilling the secret. Given that ABC is currently developing two Marvel properties for the small screen — The Incredible Hulk and Jessica Jones — we’ve got our suspicions that they might be the guest star, but anything is possible! Got any guesses, Castle fans?