The title of the show is named after the town in which the stories take place. This is Castle Rock, Maine, which first appeared in The Dead Zone in 1979.
The title of this episode could have multiple meanings. The most visceral of which involves Dale Lacy severing his own head after committing suicide.
Shawshank Prison first appeared in the 1982 novella Rita Hayworth and the Shawshank Redemption, which was adapted into a movie called The Shawshank Redemption in 1994.
The character of Alan Pangborn appeared in both novel and film versions of Needful Things and The Dark Half. In Needful Things, he was played by actor Ed Harris (who also appeared in Stephen King's Creepshow). In The Dark Half he was played by actor Michael Rooker. In the TV series, he is played by Scott Glenn.