Tigers secured top spot, with four games remaining in the Betfred Super-8s, following a 45-20 win over Trinity in front of a capacity 11,000 crowd at the Jungle.
It is the first time Castleford, who became a senior club 91 years ago, have topped the elite competition table and Powell admitted he didn’t relax until “about three minutes from the end”,
He said: “You don’t enjoy games like that too much.
“Terry O’Connor [a Sky TV pundit] said ‘you don’t smile enough’. I said ‘you haven’t been a coach have you?’ Sometimes it is hard, but I am delighted for the whole club.
“This year has been a journey and I think this is a huge achievement for us. From where we were four years ago, to being able to compete at the highest level and now winning this, we have come a long way.”
The Super League trophy will go to the Grand Final winners at Old Trafford in October and Powell conceded the job is not done yet, but he said they were entitled to their celebrations after Thursday’s win.
“We’ve got to celebrate this, why wouldn’t you?” he said. “Why would you not celebrate being the most consistent team all year?
“For me, it is an awesome achievement. The boys have been fantastic and they deserve this.”
Powell revealed Tigers will be going on a mini-camp to Newcastle next week as they step up preparations for the play-offs.
He said: “We are going to spend a bit of time with Newcastle Falcons - it is just a different environment and I think it’ll be good for us to get away and do something different.
“It’ll be good for us, we can focus on what we’ve got in front of us, which is some tough challenges.
“We can celebrate this then we will move on,
“We’ve got Huddersfield next, in a couple of weeks and when the time comes we will be ready.”
Meanwhile, St Helens and Salford Red Devils could both overtake fourth-placed Trinity if they win on Friday night, but Chester was far from dismayed by his side’s effort against Tigers.
Chester felt his team were better than the scoreline suggests and he said: “I thought we were very competitive, we did a lot of good things.
“We got within a couple of points in the second half, but then came up with a tackle five penalty and they got on the front foot and scored a try, we kicked the ball dead and they got a couple of scores on us.
“Momentum is massive in this game and once they got it we could not quite get it back, but there was a lot to be positive about. Despite the lack of field position in the first half we were still in the game and we gave it a real crack in the first 10-15 minutes of the second half.”
Chester added: “Credit to Cas. They have been fantastic all season.
“They caused us a lot of problems on our left edge and we need to fix that up, but I have said to the guys I am extremely proud of the effort they put in.”