So you watched Castle Rock and you don't understand what happened? It's OK I'm here to help. Now the creators did an explanation...of sorts, which is available if you do a quick google search, however, I felt they didn't want to give the real meaning away. They are bent on having an ambiguous ending. The problem with that is, the ending really isn't that confusing. Warning Spoilers ahead!
To quote The Gunslinger—
“Go then, there are other worlds than these”—basically there is a multiverse and other worlds/dimensions that exist. The identity of the mystery prisoner, known up until now as The Kid (Bill Skarsgård)
, is finally revealed—as Henry from another world/dimension. In one world Ruth had a son that lived and became 'the kid' in another she lost her son and adopted. Somehow both Deavers were taken out by their father to listen to the noise that he thought was God talking to him and got swapped in their respective dimensions. At the end of the episode, Holland (Henry Deaver) and Skarsgard's (the Kid/ Henry Deaver) characters go into the woods listening to the 'noise' that Hollands Deaver hears so loudly, so Skarsgard's version of Deaver could find the rift and go back to his own dimension. Holland's Deaver sees a flash of The Kid appearing as a demonic creature while out in the woods. He then decided that he must lock "The Kid' up. Now to the audience this might be seen as a delusion of the stressed out mind of Holland's Deaver, and left to the interpretation of the viewer but my theory is a bit different.
Holland has spent his life as an attorney. Not just any kind of attorney, but a criminal attorney defending those accused of murder. He learned long ago to see through people. He knows how to tell when he is being lied to. I am pretty positive he saw through the creatures lies. First of all why can't Henry remember anything that happened if he supposedly spent decades in another dimension locked up too? Particularly as he now remembers what he did to his father? So if not another version of Henry Deaver, WHO WAS THE KID??? Yeah we don't exactly get any answers. We also have no idea if Ruth Deaver (Henry's mother) actually has dementia or if she is perhaps lost somehow in time, which in Castle Rock is completely possible. What about the sound, is it God as Henry's father thought? We get no clarification on what that was all about. I'm not at all satisfied with this ending as it doesn't really settle anything
King fans have been breathlessly waiting for this series and we deserved more. Finally readers of King's work could immerse themselves into the creation of the multiverse King has built but with all new stories. I mean as the plot was unveiled with Shawshank as the backdrop for an epic tale involving a mysterious prisoner locked in the basement..you could almost hear fans squeal in delight. Unfortunately the series suffered from a plodding plot and story lines that went no where. The show never really was scary either, just really...confusing and slightly creepy.
The bright spot??? Jack Torrance's Cousin, who apparently really enjoyed hacking a guys head off earlier in the series, with an ax, just happens to be writing a book and is going to do a little research in Colorado. Ahhh.. I can already small the REDRUM. Here is hoping next season takes us to a more frightening place.