European champions Real continued their defence by beating rivals Atletico Madrid 1-0 on aggregate, Javier Hernandez's late winner in Wednesday's second leg at the Santiago Bernabeu proving the difference.
Carlo Ancelotti's men will now come up against one of Barcelona, Juventus or Bayern Munich.
And Goalkeeper Casillas is confident Real will be ready for whatever challenge is thrown their way.
"We're talking about four European champions, four top teams," he said.
"Of the most recent semi-finals, they're the most even. The semi-finals are going to be exciting, really special, and we'll play whoever is put in front of us."
Team-mate Sergio Ramos kept his preferred opposition to himself, but believes that there is no favourite to win European football's greatest prize.
"If I could choose then I'd tell you, but I won't say anything in case we get someone else," he added.
"I don't think there is a favourite. We all deserve to be this far and it is up to us to defend our title."
