Needing to beat Warrington Wolves to qualify for the play-offs, Castleford were 32-0 down in the 52nd minute but threatened the greatest comeback in Super League history when they roared back to 32-24 just 11 minutes later.
Warrington-bound Pete Mata'utia (2), Jordan Turner and Grant Millington - in his final game before retiring - all crossed to leave Wolves on the rack.
With Josh Charnley sin-binned, the visitors then also saw England prop Mike Cooper red carded for a shoulder to Liam Watts' head but, unfortunately, Castleford could not strike again.
Powell, who ironically departs to Warrington for 2022 after eight-and-a-half years at Wheldon Road, said: "That was exactly the way we wanted to play the whole of the game.
"The boys were class battling back like they did. We gave so much away in the first half and Warrington played very well.
"We showed so much courage and effort and so much skill in the second half to give ourselves a chance.
"That's what we've been about for the last eight and a half years and the boys have been unbelievable.
"We nearly had them tonight. It would have been an awesome finish. But I've had an absolute ball,"
Powell, who has coached his hometown team since May 2013 and led them to to the League Leaders' Shield, a Grand Final and two Wembley appearances, says the club will always be in his heart.
He said: "I thought it might finish it here and it is what it is.
"I've supported the club as a teenager, the club will always be special to me.
"People said they just wanted to see effort on the field when I came in but I thought it had to be about that. And we showed that."