Madrid have earmarked the 24-year-old Wales international as their major summer target although they have so far not persuaded Spurs to sell.
Madrid-based newspaper Marca's front page today suggested Spurs were holding out for a fee of 145 million euros (£126m), which would far exceed the world record £80m the club shelled out on Cristiano Ronaldo in 2009.
Whether president Florentino Pérez is willing to go to those lengths remains to be seen but Casillas, speaking on the club's tour of the United States, said: "We are talking about one of the top three or four best players in Europe and it's normal that his name is being discussed because he has all the qualities a footballer needs.
"Every transfer is controversial in its own way, for better or worse, but we are speaking about a very good, decisive player.
"I'm sure we'll make another signing before the transfer window shuts."
Casillas also gave his seal of approval to new Madrid boss Carlo Ancelotti, who took over from José Mourinho last month.
"I had the chance to meet Ancelotti during the summer holidays in Madrid and I'm excited about working with him," said Casillas, who hopes the Italian can right the wrongs of last season's trophyless campaign.
"I spoke to him for just five minutes and I came to the conclusion he is a winner. He makes that clear to you from the start.
"Whenever a new coach comes in it's a new challenge and I'm happy about starting a new era.
"Madrid need to start winning games and trophies after last season and fight for all three competitions."