The Super League leaders trailed 16-0 at half-time and looked set to lose their nine-match winning run.
But they rallied to lead 18-16 and then came from behind again to win with Luke Gale’s 79th-minute drop goal.
Asked what was said at the break, Powell said: “We needed to change a few things as it was everything; that means attitude.
“We needed to show we are the team we think we are. That’s not taking anything away from Wakefield who dominated us and were too quick for us.
“But the main thing was it was our attitude not quite being where it normally is.
“It was a great second half effort, though, to reel it back in. It really was.”
Powell maintained his side did not under-estimate Wakefield who could have gone joint-fourth with a win.
“We’ve won nine game straight so it’s human nature that now and again you dip a little bit,” he said.
“I don’t think it’s underestimation – just natural – and we needed to rectify that.
“We’re mindful of it and the players are too; there’s a lesson there for us that every week it’s on.
“We still had to win it twice in that second period, too. The second half was a great example of how to win when under pressure. Everything was better.”
Wakefield boss Chris Chester saw his side lead 24-18 when David Fifita crossed in the 69th minute and Liam Finn added a penalty but Jake Webser struck back to level.
“I thought we did (deserve a draw),” he said. “It was a tale of two halves and the first was probably the best 40 minutes we played all season. We really took it to their pack, put them under pressure and were very aggressive from kick-off.
“But credit to Cas. They are a class side and it shows how far we’ve come when we’re disappointed to lose to them. They’ve put sides to the sword before.
“When big Dave (Fifita) went over I thought we were in for a good night.
“We’re very disappointed to lose in that manner. I’m proud of the efforts. I never question that. We just needed to be a little bit smarter when we went in front near the end.”