The Black and Whites won 40-14 at the KC Stadium, booking a last-eight home date with Leeds Rhinos.
“The finish was really pleasing,” Radford said, according to his club’s official website.
“We spoke all week about playing for 80 minutes. Every time we’ve played against Castleford under Daryl Powell in the past we’ve had to put in 80 minute performances.
“For long periods in the game the momentum was not with us so to hang in there like we did was really pleasing.
“We left some players on for people longer than we would have liked because we had so many forced interchanges but I thought we did a fantastic job and scored some great tries.
“We are playing a little bit more with the ball and we are more confident in our defence.
“We’ve knocked off some pretty formidable opposition in the last five or six weeks and we’ll be playing the Champions on Friday. We now have to prepare for that.”
Meanwhile, one hero for FC in their fine win over the Tigers was utility back Curtis Naughton, who bagged a hat-trick of tries.
The 20-year-old would obviously be delighted if the Airlie Birds made it all the way to Wembley.
“It’s everyone’s desire to get to Wembley,” he told FC Live.
“It’s one dream of mine to play there and hopefully that can come true.
“All I want to do is play in competitions like the Challenge Cup and lift trophies.
“For now, I’m always trying to improve and I’ll keep doing that in training.”