After the transfer saga of Argentina's Tigre keeper, Daniel Islas, on 7 January 2011, was officially announced that Castillo signed a two-year contract with Colo-Colo. In his arrival he was assigned with the kit number 1, previously worn by Francisco Prieto, the club's 2010 first choice keeper marginated by coach Diego Cagna to face the season. At Chilean club, Castillo was reunited with his national team teammate Andrés Scotti and on 9 January joined the club's pre-season in La Serena alongside Colo-Colo's another signing Agustín Alayes where he declared: "It's a joy arrive to the most successful team of Chile", finally adding: "I'll defend this team with much sacrifice and pride". Castillo made his friendly debut against Deportes La Serena in a 1–0 loss with a score of Javier Vatter in the 75th minute. After their stay in La Serena, Colo-Colo continued the pre-season at Santiago and in the following match against Unión Española he stopped a penalty to Sebastián Jaime in a game that finally Colo-Colo won 3–1. On 26 January, during the Noche Alba (Colo-Colo's last pre-season game where presents their signings) he produced a man of the match performance in a 3–2 defeat to Nacional, so that won the recognicement from the press and the fans despite the loss.