It's a good offer and, after giving it some thought, it is something I want to follow up," Castle told the club's official website.
"I've really enjoyed my time here and I wish the club every success in the future.
"The supporters here are fabulous and they deserve success and to be back in the Football League.
"It was a hard decision to make, but this is something I've always wanted to do."
Power added, "We thank Steve for all his efforts here this season and wish him success in his future endeavours.
"Jimmy has enjoyed working with Steve, but when he announced that there was an avenue he wanted to follow we didn't feel it would be right to stand in the way of him furthering his career in other ways."
Source: http://www.confguide.com/newsArticle.do?id=6387