A former first-round pick by San Diego, Castillo had drawn interest from multiple teams since the start of free agency, including the Patriots and Chiefs, but ultimately decided to return to the only franchise he has played for as a pro.
Originally the 28th overall pick in the 2005 draft out of Northwestern, Castillo has 210 career tackles to go along with 19 career sacks.
An opening-week injury limited him to just one game in 2011, but he'll return to San Diego with a chance to prove that he can recreate the form that once made him a defensive pillar for the Chargers.
At 6'3" and nearly 300 pounds, Castillo has played a consistent role as a 3-4 defensive end in San Diego. As Kansas City and New England both play a steady dose of 3-4 defense, this probably contributed to their interest in his services.
Castillo made news around league circles before even entering the NFL when he admitted in a letter to all 32 franchises leading up to the 2005 draft that he had previously taken steroids (via ESPN). That didn't prevent San Diego from taking him in the first round, although it remains a talking point about him to this day.
Castillo, a Brooklyn, New York native, will be 30 years old at the start of the 2012 regular season.