Madrid made it three wins from three in Group B games with a 2-1 victory at home to the Turin side on Wednesday night and one more point would all but guarantee progression.
The hosts made hard work of it at the Bernabeu, playing against 10 men for almost the entire second half following Giorgio Chiellini's red card.
Casillas told the Madrid website: "We have taken an important step towards qualifying for the next round.
"It was not easy, even though they had 10 players for some of the second half.
"We have nine points from three matches and now we are going to Turin with the idea of at least not losing, which would be a good result and put us through to the next round."
Madrid already led 2-1 when Chiellini was dismissed and Sergio Ramos admitted the team took their foot off the gas in the second half.
He said: "The result might have seemed easier after their sending off and the team may have relaxed. That is something that we have to improve and hopefully this will help us reach our best."