The 23-year-old Davies, who had a seven-game spell on loan with the Tigers in 2011, has joined them until the end of the season after being released by Widnes this week.
"I'm delighted to have Ben on board," said Powell. "We've got some key games coming up and he's really going to help. He brings aggression, go-forward and he's got a good skill set."
He added: "I've spoken to people about the last time that he was here and everybody was delighted with his impact so I think all those things that he will bring to the pack are much needed."
Davies made 24 appearances for the Vikings in their first season back in Super League in 2012 but has played just once this year and in April joined South Wales Scorpions on a month's loan.
Davies said: "It's good to be back. As soon as I heard that Cas were interested in me I jumped at the chance.
"I've got the bit between my teeth about coming here and doing a really good job for this season and hopefully beyond.
"I feel I've got a point to prove. Last year I felt that I proved that I was more than capable of playing Super League every week, even though I'm only 23.
"To have it cut from beneath me this year was really frustrating so I want to play every week and show people what I can do and prove my worth."