La Roja's No. 1 believes every team will be out to upset the reigning champions and lamented the draw process which saw his side pitted against Netherlands, Chile and Australia
Vicente del Bosque's side secured its country's first-ever senior global crown in South Africa in 2010 and has since secured back-to-back European Championships after thumping Italy in the 2012 final.
However, Casillas believes that it will be a lot tougher for la Roja to retain their title in Brazil, with a number of sides eager to claim a memorable scalp against the 2013 Confederations Cup runners-up.
"We know that it'll be very difficult because everyone wants to be champions, not just Spain," the Real Madrid keeper told reporters at a special press conference to discuss the tournament in Brazil next year. "After the last four years we know that every team will give 100 percent against us and will want to beat us. But we go there with the aim of defending the title which we won in South Africa and try to make the people of Spain proud again.
"We can promise that we will make a huge effort. We will be up for it, and we are full of hope that we can do it."
The Real Madrid captain confessed he was also a little aggrieved to find his side pitted in a daunting group along with Chile, Australia and 2010 finalists Netherlands.
"I agree the draw is sometimes hard to understand, especially the coefficients, but never mind, we are in that group. We don't choose, all we can do is turn up and play and hope to win and congratulate the winner," he added. "But it's true that I don't agree with how the draw has been done. In any case, we aren't the ones that take these decisions, but we are not qualified to say which team is better than others."