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"
post #2593 Q confirms red/green castle is the Corp of Engineers who are building the southern wall. COE falls under the military command and when Trump could not get money in the government funding bill due to Dems not wanting a wall built, Trump went around them and had the COE build it. Now post #4414 has "castle_rock" in it which means Trump just visited the southern border wall which was just completed. Post #4414 is a road map for the next few days. Once all the pieces have been done/accomplished then the BOOMS can begin. Hold tight, between now and the 20th things will get exciting. Thurdsay and Friday are good days to round up bad actors in Congress. Monday is a holiday, tuesday nobody will be in town and Wed is inaguration day. Might not have too many pols in town due to mil build up. Hopefully 1/14 or 1/15 will be the days something big will happen.