Real claimed a 9-2 aggregate victory over Schalke in last year's round of 16 on their way to lifting the trophy.
However, Casillas is not expecting an easy ride this time around against the Bundesliga's fourth-placed side.
"It will be very difficult," he is quoted as telling UEFA. "The atmosphere in Germany will be very intense.
"They are aware that they have an opportunity to get revenge and we expect a tough game.
"The fact that we faced them last year does not benefit us. If anything, it gives them an advantage.
"It would be a mistake to think that playing in Gelsenkirchen will be easy and that we'll repeat what happened last year.
"Quite the opposite: we have to be careful and go there with humility. It will be a very tough tie."
Ahead of the trip to Germany, Casillas praised the way one of Real's own German stars, Toni Kroos, has settled into the club.
"This season, some players who were important to our style of play left the club," he added. "Others arrived and have become key elements of the team, like James [Rodriguez] or Toni Kroos. The statistics are there to see and demonstrate that.
"Kroos feels very comfortable with the ball. He looks to move it, pass it quickly from right to left."
"He's quick and young. He has being playing to a top level for a long time, so I think that Real Madrid were right to sign him, above all when you take into account his age.
"He has already shown that he can be key in the midfield."
