They will look to lift the trophy for only the fifth time in the club’s 95-year history when they face St Helens at Wembley
Castleford came close when losing to Leeds Rhinos in the 2014 final, Powell’s first full season in charge at Wheldon Road.
He has helped transform the club during his tenure but will leave at the end of the season to take over at Warrington Wolves, with plenty of this current crop of players also expected to depart.
Powell - who won the Cup as a player with Leeds Rhinos in 1999 - admitted: "If I could have said right at the start of the year what could be the most special thing that could happen this year it would be winning the Challenge Cup.
"It's an iconic competition. It's iconic in Castleford's history.
“We look back at ‘35, ‘69, ‘70 and ‘86 - the times the Cup’s been at Castleford - and we want to leave our own legacy.
“For me it would be super special. It’s my last year at the club and I can't put into words quite how much that would mean.
"We want to be able to help our supporters remember it forever and you remember it forever if you win it.
"Wembley is embedded in the sporting psyche and history of the club.
“You look at last night and the England game, Wembley means something completely different to people.
"It's such a special time for fans, walking down Wembley, it's a feeling like nothing else. It's just unbelievable as a sporting showpiece.
“I’m sat here surrounded by pictures of Alan Hardisty, Mal Reilly and John Joyner with the Cup. We want to make our own memories.”
Powell says he has learned from mistakes made when they lost in 2014.
"I think I tried to play it down and keep a lid on it," he said.
"In hindsight that was a bit of a mistake.
"We want to enjoy the experience and be excited by the challenge.
“That's one of the biggest messages I've given to the players this time. We're going to enjoy the week and everything that it stands for."
Having rested the majority of his likely Wembley line-up for yesterday’s Super League game against Salford Red Devils - Castleford lost 70-18 with five teenage debutants in the side - Powell hopes to have kept his players fresh for Wembley.
Australian prop Grant Millington successfully came through yesterday’s game, his first since injuring a knee two months ago, and had a light run-out with the squad this morning as he looks to prove his fitness in time.
Powell’s major doubt is stand-off Jake Trueman who has not played since the semi-final win over Warrington Wolves due to a recurring back issue.
"He had a run today and looked better," said the coach.
“We'll see how he goes through the week. At the moment, he's 70-30 (to play) I would say."
He could select from a fully-fit squad aside from Sosaia Feki, the winger who was ruled out for the year when rupturing his Achilles in pre-season.
“We’ve got a couple of checks (on players) as we roll through the week,” said Powell, who also confirmed the entire squad had passed their latest Covid-19 tests today.
“I’ve a couple of calls to make as you usually do in finals which are really difficult. I’ve just got to make sure I get those right.
“There’s a great opportunity for us. Everyone will have written us off, I’d have thought, but we’re looking forward to the challenge and we’re excited by it.”