The club captain is adamant that Los Blancos have passed an important test against Pep Guardiola's men en route to the Liga title as he praised his team-mates for a job well done
Jose Mourinho's side are now seven points clear of the Blaugrana at the summit of La Liga with only four matches left in the league calendar, and the Real captain is the latest player from the Blancos camp to share his optimism the win has given him for the rest of the season.
"We've taken a gigantic step in La Liga by defeating Barcelona," said Casillas after the match.
"There are still points in play and we must show respect for our opponents, but winning here is important step in the championship."
Casillas then singled out match-winner Cristiano Ronaldo for special praise but commended the entire side for their hard work.
"We behaved like a true team. Cristiano did an excellent job, but all of us played well from start to finish. It was a very important match and we earned a difficult victory.
"We worked well throughout the game, but we suffered after they scored the equaliser. Nevertheless, we reacted and won."
Madrid now turn their attention to the Champions League to overcome a 2-1 deficit when they welcome Bayern Munich to the Santiago Bernabeu on Wednesday for the second leg of the semi-finals.