In a wide-ranging interview with Spanish newspaper Marca, the Spain keeper said Madrid will always be his home but that he would consider a transfer to another top European league.
Arsenal transfer news
Arsenal were heavily linked with Casillas over the summer, and they signed David Ospina to back up Wojciech Szczesny. Liverpool have reportedly been in contact with Casillas's Spain teammate Victor Valdes, as he's currently out of contract having left Barcelona over the summer.
He was supposed to be joining Monaco but that fell through after Valdes suffered a serious knee injury. He's nearing a return to full fitness now though, and Liverpool are the frontrunners to sign him on a free transfer to compete with Simon Mignolet.
As for Casillas, the Spaniard may join his international teammate in moving abroad if he fails to win back his regular place at the Bernabeu.
Thought about departure
"I thought about leaving," Casillas told Marca. "Much has been made about an offer from Arsenal, but there was nothing concrete.
"Now I want to stay at Real Madrid and end my career here."
But Casillas understands that may not be possible as the reigning European champions signed Keylor Navas over the summer.
"I would like to experience [a league] abroad before I retire," the 33-year-old keeper said. "I think that would help me appreciate Real Madrid even more.
"I am convinced that if you leave, you realise what it really means to represent this club [Real Madrid]."
Arsenal keeper Szczesny is the undisputed number one at the Emirates Stadium, but concerns remain about his consistency and his long-term suitability for a club that wants to challenge for a Premier League title.
Arsene Wenger has great faith in him though, which means a move for Casillas may be unlikely in the January transfer window - unless Liverpool cannot agree a deal with Valdes.