The 29-year-old Spanish international's consistent performances have attracted much interest, including from both Manchester United and City.
Although he has been at the Spanish giants all his career and is happy at present, he does not rule out moving on some day.
This may come sooner than later if a big-money bid comes in for Casillas, especially as new Madrid boss Jose Mourinho has revealed he wants to buy a top-class keeper this summer.
Casillas, who is currently on duty at the World Cup, told the Daily Star Sunday: "I am aware of the interest of both Manchester United and City.
"I'm always flattered whenever another club shows interest in my qualities as a goalkeeper. I have been at Real Madrid all my career and I still enjoy going to training every day.
"I enjoy meeting the fans too - but we all know how uncertain things are in football.
"I'm not going to say never. Right now Madrid would have to kick me out of the club before I consider a move.
"Who knows what will happen, though, if a big money offer comes in."