The Tigers were trailing by 10 points at half time in their new coach's first home game in charge of the side, but claimed a slender win thanks to a second-half comeback.
Powell told Sky Sports News: "It was unbelievable to be honest. You didn't know which way it was going to go from start to finish.
"I'm delighted for the team and delighted for the fans. It was good to see them at home at last. I'm obviously pleased to come out as winners but there are certainly some things we need to look at that we need to improve.
"We're trying to cut down on the number of points we concede but we worked so hard."
Castleford sit 12th in the Super League table.