Full-back Luke Dorn, 33, scored a hat-trick of tries in his first game in two months and captain Luke Gale was superb at scrum-half as Castleford heaped misery on their derby rivals, who remain bottom of Super League.
“I’m just really proud,” said Powell.
“I’m delighted we’ve been able to put together a complete performance like that.
“Our attacking game was sublime at times and I thought Luke Dorn back in the side was outstanding while Luke Gale was pulling strings, too.
“Whoever played at half-back with him did a great job as well, but it was probably defensively as well that impressed me.
“I just thought we got after Leeds; we were really tough, took ground off them consistently and that attitude was really pleasing.”
It was their biggest win at Leeds and most impressive since 36 years ago when Malcolm Reilly, such a big influence on Powell, was in charge of Castleford.
With Tigers up to sixth, the coach added: “It’s been building.
“Obviously getting Dorny back gave us some impetus, Junior Moors’s game is building all the time and our pack is really aggressive and tough at the moment and that’s been building as well.
“There’s just a confidence about us at the moment.
“(Last night) was a big effort – Leeds are obviously struggling at the moment and I do feel for them but I was really proud of how we went.”
Sam Tomkins has pledged to provide Wigan with the spark they have been missing when he makes his eagerly-awaited return to Super League tonight.
The England full-back, who has re-joined his home-town club from New Zealand Warriors on a four-year deal, admits it will take him some time to find his form after a nine-month injury lay-off, but he intends to make a mark against Hull in the top-four clash at the DW Stadium.
“I’m realistic,” Tomkins said. “I’m not going to be coming out Friday and performing to my very best. I know I’ve had a significant injury and getting back to playing is not about your best form immediately coming out. But I’ll certainly add something to the team on Friday, I’m not going to be a weak link, that’s for sure.”