Head coach Rhys Morgan's right hand man knows Castle from the time they spent together at Cleckheaton and former Heath skipper Harrison is in no doubt that the playmaker is a more than useful acquisition.
"The signings of Dom Castle and prop Paul Turner are probably the best this club could have made during the close season," aid Harrison, who lifted the Powergen Junior Vase at Twickenham as skipper of Heath in 2002.
"Dom has played at a very good level with Wakefield and spent a season at Morley before joining myself and John Bentley at Cleck.
"He is a very knowledgeable player with bags of experience. He is a tremendous leader on the pitch and it is like having an extension of the coaching team out on the pitch which can prove important.
"Also, Dom's service is very good and he is a very brave player in defence.
"I know Craig Aikman was a very influential player when he was at Halifax, but Dom has the potential and the game to be just as effective."
Harrison reported that pre-season training had been going well and that the club were still actively looking to increase the number of players.
Top of the list of priorities would appear to be a prop forward with Turner the only fully fit front row forward on the books.
"Adam Blades and Euen Griffin have both had minor ops so they aren't fully involved in training at the moment," added Harrison.
"Griffin has had an operation to clear out his knee while Adam has also had a knee and groin op.
"Hopefully it won't be too long before they are able to train properly but even so we are probably still looking at mid-August for the pair of them.
"We could also do with another midfield back to be honest.
"We have irons in the fire and we are always looking to strengthen."