Segundo Alejandro Castillo Nazareno is an Ecuadorian professional footballer who plays for Guayaquil City of the Ecuadorian Serie A. Castillo is mostly used as a defensive midfielder but can also play as a central midfielder. Between 2003 and 2016, he made 87 appearances for the Ecuador national team scoring 9 goals.
Castillo broke into the Ecuadorian national team in 2005, making his international debut on 17 August as a substitute against Venezuela. He became a regular player after this and was included in their squad for the 2006 FIFA World Cup. He played every minute of their World Cup campaign, where Ecuador reached the second round for the first time in their history. He has gained a starting role since the 2006 campaign, mainly as a defensive midfielder and has captained the team in a few occasions.