After outpointing Ahumada over six rounds in a rematch held in Phoenix, Castillo won three consecutive fights by first round knockout, including his first fight outside Arizona, when he beat Regis Rodriguez on March 31 in Philadelphia, Pennsylvania, and his Las Vegas debut, on April 16 against Juan Aguilar.
Part of his popularity was due to his having fought many of his early bouts in places with large Chicano and Mexican populations.
On September 17, 1976, Castillo won the Arizona state Featherweight title by outpointing Ahumada after twelve rounds in their third bout, held in Phoenix.
Castillo advanced to the US tournament's finals after outpointing Walter Seeley in ten rounds, April 2 of 1977 in San Antonio, Texas.
Castillo was undefeated in 36 bouts, with 18 knockout wins, when he challenged James Martinez for the USBA featherweight title on June 15, 1979, in Las Vegas.
Castillo conquered the United States Boxing Association's title with a twelve round decision win over Martinez.
His next fight took place in Brazil: trying to keep a privileged ranking among Jr. Lightweights, and to obtain a rematch with Chávez, Castillo lost a ten round decision to Tomas Da Cruz on August 18 in São Paulo.
Castillo, by his part, fought once more, beating Martin Morado by a ten round decision on March 4, 1986, then announced his first retirement.
In 1989, he made an unsuccessful comeback bid, losing two fights, and requiring hospitalization after being knocked out in the fifth round on August 30 by Edgar Castro.
After retiring for the second time, Castillo was active as broadcaster for ASPN, analyzing fights in many places across the United States' southwest.
He won five fights and lost one (to Fabian Tejada, a well known Argentine boxer of the era), before challenging for the regional, NABO Lightweight title.