Sports Director Rouven Schroder added: "Castello has matured into one of the best defenders in the Bundesliga in just one year and has played a major role in our defensive stability this season. He can defend toughly and also play a key role in building up play. His strength in tackles, his good passing, his confidence and composure on the ball, his speed and his flexibility in being able to play both as a centre-back and as a full-back make him an exceptional player. We are delighted that Castello has committed himself to RB for the long term and sees his future in Leipzig."