I have been looking at this for the last few days and have a possible decode. The underscore could be designating a missing letter. As Q has done in the past. So I started by looking for words that are scrambled within the main body of CASTLE ROCK. Being that this is in a context of part of a time line one word stands out. "CLOCK" which leaves " _ASTER". There are several words that you can make out of this using different letters for the underscore but ONE stood out to me using the letter "R".
"ARREST CLOCK"
I dont think Q gives us puzzles, I think Q gives us riddles.
To expand on that, He makes us ask or answer the important questions.
My current idea is CASTLE_ROCK is the pentagon/Military.
Post 4413 https://www.axios.com/tina-kaidanow-fifth-pentagon-official-resigns-4ea23438-d930-4e6b-be1e-9e370492e385.html
https://twitter.com/Calloutfortruth/status/1268642419635114000
Those loyal to our Constitutional Republic and our Command Structure.
Those loyal to the Office of the President and the will of the people.
Those who swore an oath to protect and defend against all enemies foreign and domestic.
Those good people who serve proudly for America.
CASTLE_ROCK.
FOR GOD AND COUNTRY.
Q
See my comment on this