The Primeira Liga outfit are well-placed to reach the UEFA Europa League semi-finals ahead of Thursday's clash at the Estadio Ramon Sanchez Pizjuan after winning the first leg 1-0.
Despite being without suspended star striker Jackson Martinez, Castro said Porto would attack without losing their shape.
"A play-off match that ends 1-0 in its first leg gives strong hopes to both teams," Castro told a news conference.
"I expect Sevilla to try to win the match but they can expect the same thing from us. We'll try to win in a balanced way, trying not to forget our defensive system.
"We're a very determined and brave team. We know that a goal can give us more stability in the game and that's what we are going to try from the first minute. We'll definitely be trying to score."
Castro defended Martinez, who has starred with 18 league goals this season to be the Primeira Liga's top scorer.
Martinez picked up a 42nd-minute yellow card in the first leg to be suspended, while Fernando is also out after his red card.
"One thing I always defend as a coach - I think the players have to go to the pitch to play with intensity, with love," Castro said.
"In Jackson's case, especially, it makes no sense to talk about the yellow card because the yellow card shown to him made no sense.
"At the end of the match I was asked about the referee and told that if I was him I would not have shown the card to Jackson."
