Levante return to the top flight this weekend at Espanyol, and Castro was clear about his priorities.
"The objective is to beat Espanyol," he said. "You have to go into the game focused because you don't know what will happen afterwards. Pre-season has given us good feelings, but we're not at 100% of what we want. Some players arrived late and others who were important last year have left.
"But the group is working very well and taking on board what we want. At this stage we're correcting details, and the big principles are already understood."
The Portuguese coach addressed the sale of forward Carlos Espí to Real Madrid, the club's biggest departure of the summer.
"We wouldn't have sold Carlos Espí if it had been up to us, but it's something that had to be done," Castro said. "We know the club has had problems for many years that are being fixed little by little, and that's why we make sales that are necessary."
He rejected any suggestion of over-reliance on the striker.
"I like Espí a lot, but we didn't have an 'Espí-dependency'. If the team wasn't functioning well, the ball wasn't going to reach him. I remember clearly that first we had a problem where we weren't winning at home, then we conceded too many goals, then our forwards weren't scoring. We always had problems, but I said we had solutions.
"It's the same now. In pre-season we've scored goals, and we've scored them without Espí. The squad is ready to win. He's a very good player, but we also have players who can surprise this season."
On squad registrations, with only 12 players currently eligible for La Liga action, Castro said he had prepared for the opener "counting on everyone," adding that the club was doing "everything possible" to resolve the situation, even if that meant further sales. He described the summer's business overall as "very good" given the budget available, and said any further incomings would depend on outgoings, though the club was "prepared in all positions."
Asked about individual players, Castro said Karl Etta Eyong would be central to his plans without the team relying solely on him, recalling his matchwinner against Villarreal last season: "We won 3-2 with a goal from him, and if he hadn't scored we'd be in the second division now."
On Carlos Álvarez, he denied speaking to anyone about a sale: "He's working well and giving his all. If there's a day he has to leave, he will — nobody is eternal at a club."
Xavi Grande was praised as a quality option but currently behind Toljan and Nacho in the full-back pecking order, while new captain Aïssa Mandi was described as an obvious choice "for anyone who watches his daily work."
Marc Santos was confirmed as part of the first-team squad "like everyone else," and recent arrival Thiago Fernández was said to be physically ready but not yet prepared to start.
On Espanyol themselves, Castro was wary of the opposition's stability.
"They haven't changed much. They haven't sold any important player except Romero. They didn't lose by five goals at any point last season, which tells you how competitive they are, and they haven't changed manager either."