According to The Baltimore Sun, Castillo has been spending time with the team in recent weeks in preparation for his new role.
He will be a full-time member of the staff for 2013.
Castillo was dismissed as the Eagles defensive coordinator following the team's disappointing 26-23 loss to the Detroit Lions in Week 6, a game in which the Eagles blew a 10-point lead in the final four minutes of regulation.
Castillo had been promoted to the role of defensive coordinator before the 2011 season, despite never coaching defense at an NFL level. In fact, Castillo hadn't coached on the defensive side of the ball since he was a defensive coordinator at Kingsville High School from 1986 to 1989.
The Eagles offense struggled in 2011, blowing five fourth quarter leads. They did improve at the end of the season, allowing just 46 points during a season-ending four-game winning streak.
The defense started strong in 2012, holding off Brandon Weeden, Joe Flacco, and Eli Manning in a trio of late-game defensive stops that resulted in victories for the Eagles. But Ben Roethlisberger and Matthew Stafford engineered walkoff victories against the Eagles, ultimately sealing the fate for Castillo in Philadelphia.
The previous 13 seasons, Castillo had worked as the team's offensive line coach, where he helped develop Tra Thomas, Jun Runyan, Jermane Mayberry, Shawn Andrews, and Jason Peters into Pro Bowl players.