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"
Well, Q has many times written C_A, where the undersore represents letter "I". So, I guess, CASTLE_ROCK could be CASTLEIROCK, which is an anagram of SALTIER COCK and REALIST COCK. (Apologies to the ladies.)
But then, it could also be "RIOT CACKLES" or "ALSO CRICKET", which throws an enirely different light on it because they play cricket mostly in England, India and Australia.
Hmm.