The experienced goalkeeper was not too disappointed with his side's draw against BVB at the Santiago Bernabeu and opined that the Blancos can still progress to the next round
The Spanish champions currently sit second in the standings with seven points from four games, and defend a three-point lead over third-placed Ajax with two more games to go.
"There is one game less to play and it is still down to us [in terms of advancing]. We're only one point behind Dortmund so we are still going to fight for top spot. Our group is a difficult one that contains important teams," Casillas told Uefa.com.
"We knew that if we had won today it would have meant a big step towards qualifying but we must take the positives. We have managed to save something from a game when we were behind.
"Our opponents tonight [Tuesday] were very good in both games we played against them. It's strange that in Germany they [are fifth in the standings] whereas in Europe they are playing like that.
"In the first half tonight they controlled the ball better and managed to get through to our area. We went looking for the equaliser in the second half and it eventually came. We hardly conceded a chance after the break so I think that the draw tonight is fair."
Real Madrid resume domestic action on Sunday when they travel to Levante.