Castaño won a South American Games gold medal and tallied a 181–5–5 record as an amateur,[1] with wins over Errol Spence Jr. and Esquiva Falcão.

Castaño participated in the 2009 World Amateur Boxing Championships, winning his first two bout before losing to Jack Culcay-Keth.

Castaño accumulated a 3–0 record in the World Series, including a famous win over Sergiy Derevyanchenko, who had previously been unbeaten in the competition.

The judges seemed to prefer Castaño's activity and consistency over Soro's calculated pressure, who admitted he had started slow in a post-fight interview.

Castaño showcased his talent, and defeated Vitu via a 12th-round TKO, dominating him from start to finish to retain his WBA (Regular) title.

[11][12] On June 25, 2019, the WBA announced that they are stripping Castaño of his belt, because he did not sign the contract for a rematch against Cedric Vitu, his immediate mandatory challenger.

[14] On 13 February 2021, Castaño defeated WBO light middleweight champion Patrick Teixeira via wide unanimous decision with judges' scores of 119–109, 120–108 and 117–111 in his favor.

[16] The result was controversial, with much attention being brought to judge Nelson Vazquez's 117–111 Charlo card, which was described as "terrible" by Andre Ward.

[21] A day later, the WBO ordered Castaño to show cause as to why Charlo should't be rescheduled to make an overdue mandatory title defense against Tim Tszyu.