Castillo is a strong two-way player with the ability to control two gaps and penetrate into the backfield. However, he has not lived up to his ability as a pass rusher since his sophomore season in 2006. Ron Rivera will do more to free up Castillo this year, but the onus is on the sixth-year veteran to justify the team's investment in him.
Likely to start on the opposite side of the line is Jacques Cesaire. A former undrafted free agent out of Southern Connecticut State, Cesaire enters his eighth season having played in all but three games over the last six years, starting 48 of them. He is an active, disciplined defender, but he is stronger when his snaps are limited.
The battle for playing time among the backup ends is intense. The first end off the bench last year was Travis Johnson, a former first-round pick of the Houston Texans who came over in a preseason trade. Johnson played through a nagging groin injury for much of last season but still finished with 13 tackles. He should be better now that he's healthy and more comfortable in Rivera's defense.
DE Alfonso Boone
The Chargers were forced to sign Boone last season because of a rash of injuries on the defensive line, including a triceps injury that cost Ryon Bingham his '09 season. Bingham is healthy again now and ready to resume his spot in the rotation. Prior to getting hurt, Bingham was seen as an ascending player who could push Cesaire of the starting job at right defensive end. A versatile defender who could also offer some snaps at nose tackle, Bingham is particularly strong against the run.
The X-factor in the mix at end is Vaughn Martin. A fourth-round pick in 2009 out of Western Ontario, Martin's rookie season was essentially one big learning experience. After some struggles early on, he played his way back into the rotation and provided some quality snaps, especially in short-yardage situations. He is the best athlete on the defensive line, but he'll have to turn potential into production to survive.
The logjam at end is staggering, but the picture at nose tackle isn't much clearer.
One player who is seemingly a lock for a roster spot is rookie Cam Thomas. The Chargers moved up in the fifth round of this year's draft to pick the massive nose tackle and he has not disappointed. He can anchor the middle of the defense and prevent interior offensive linemen from reaching his linebackers. The Chargers will limit him to about 25 snaps per game to maximize his effectiveness, but he has earned his place in the rotation.
Thomas will play mostly on first and second downs, but indications are the Chargers prefer a veteran in the starting lineup. In the preseason opener, that veteran was second-year nose tackle Ogemdi Nwagbuo. The former undrafted free agent from Michigan State came on strong during last year's training camp. He won a roster spot as a defensive end, then slid inside due to injuries. He has the quickest first step of all the interior players, but his lack of size is concerning.
Antonio Garay, like Boone, was a midseason pickup in 2009. Don Johnson prepped him to play nose tackle after Nwagbuo went down with a season-ending ankle injury and the team was thrilled with Garay's progress. He was running with the first-team defense for most of the offseason, which is why it was somewhat surprising he didn't get the start against the Chicago Bears. Nonetheless, his blue-collar approach and ability to stay off the ground give him an excellent chance to stick around.
DE Derrick Jones, a first-year player from Grand Valley State, is also on the roster. His best-case scenario is a spot on the practice squad.
Which linemen will make the final 53-man roster? Discuss in the message boards.
Michael Lombardo is a member of the Pro Football Writers of America and a long-time contributor to the Scout.com network. His analysis has been published by the NFL Network, Fox Sports and MySpace Sports. He has followed the Chargers for more than 15 years and covered the team since 2003. You can see more of his updates by following him on twitter.