Early career

Castillo made his professional debut in early 1997 in a tournament. He won his debut via rear-naked choke submission only 40 seconds into the first round. He won his next fight after his opponent broke his wrist during the fight, and in the tournament final, Castillo faced veteran Vernon White and won via armbar submission in just three minutes. Castillo fought for the King of the Cage Middleweight Superfight Championship on April 29, 2001, against then-champion Joe Hurley. In a back-and-forth fight, Castillo scored seven takedowns on the Lion's Den fighter, and won via unanimous decision. Castillo then fought future Strikeforce Welterweight Champion Nate Marquardt for the IFC Welterweight Championship and won via unanimous decision before being invited to compete in the UFC.^[5]

UFC

At UFC 33 on September 28, 2001, Castillo made his UFC debut, fighting a much-larger Dave Menne for the first-ever UFC Middleweight Championship. Castillo lost via unanimous decision and was handed his first professional loss.

After picking up a unanimous decision win over Chris Brennan at UFC 35, Castillo fought for the UFC Welterweight Championship against then-champion Matt Hughes at UFC 40 on November 22, 2002. Castillo lost via doctor stoppage due to a cut received in the first round.^[5]

WEC

Castillo then began fighting for the WEC, making his debut for the organization on March 27, 2003, at WEC 6 against Chris Williams. Castillo won via TKO.

Castillo fought on May 21, 2004, at WEC 10 against future IFL Lightweight Champion Ryan Schultz and lost via TKO. His last appearance for the organization was on October 14, 2005, at WEC 17 against Steve Ramirez. Castillo won via TKO. Castillo's fought last on April 1, 2006, in an IFC event against current UFC Welterweight Jake Ellenberger. Castillo lost via TKO in the first round.^[5]