Leverkusen are currently occupying third spot in the German league with a four-point lead over Frankfurt in fourth, and one point off champions Borussia Dortmund in second.
But with 11 games left in this campaign, Castro admits that every game from now until the end of the season will be vital to securing qualification for the Champions League.
“We now see every game as a final and want to secure third place," he told reporters.
“The Champions League is the big thing," he continued.
"This is for each of us - fourth place would be for me a big disappointment."