Barcelona and Olympique Lyon will take healthy leads into the second leg of their last 16 clashes against Bayer Leverkusen and APOEL Nicosia.
The reigning champions netted three away goals at the BayArena, with Alexis Sanchez scoring his first goals in the Champions League, and Lionel Messi volleying home a late third.
Michal Kadlec had equalised for Leverkusen, but despite Castro striking the post, Leverkusen were helpless to stem the Barca tide.
Midfielder Cesc Fabregas said: “There were reasons we played like we did in the first half: they are a very physical team. Perhaps this kind of play is a bit boring but that’s the kind of play we needed to employ.
“We’re always excited [to play] even though it can appear otherwise from the outside. The team always has the same level of excitement and the same motivation. The boss won’t let us give up.
“They can complicate the whole game with one counterattack but we dominated the situation. We’re very pleased.
“What happened in Pamplona was a disappointment because we let the league slip away, but the Champions Leauge is the most beautiful competition there is. The Champions League is difficult [to win] because the best teams in Europe compete for the title.”
In France, Alexandre Lacazette's looped finish gave Lyon the advantage over APOEL, representing Cyprus for the first time in the knock-out stage of the Champions League at Stade de Gerland.
Despite qualifying top of Group G, APOEL are the rank outsiders of the knock-out phase, and having manufactured one shot on goal throughout the whole 90 minutes, will need to improve on their first-leg showing.
Kim Kallstrom, the Lyon midfielder, says his side have given themselves a great chance of making the last eight.
"We turned in a really good performance," the Swede said.
"We were balanced overall and dangerous in attack. The one thing we lacked was a second goal, but we aren't going to complain.
"We've put ourselves in to a strong position to go through."