Casteel began his career as a defensive assistant at California University in Pennsylvania, his alma mater, where he spent three seasons. In 1987, he left to Miami to be the defensive coordinator at Palmetto High. His next move was to Shepherd College in Shepherstown, West Virginia, where he served as defensive coordinator and strength coach from 1988 to 1999.
He left Shepherd to become defensive ends coach at Texas El-Paso in 2000, and in 2001 he was hired as the defensive line coach at West Virginia for Rich Rodriguez's first season. He occupied that position for one season before being promoted to co-defensive coordinator, a position he shared with now Arizona State head coach Todd Graham, and linebackers coach in 2002.
In 2003, when Graham left West Virginia to take over as the defensive coordinator at Tulsa, Casteel was promoted to his current position as defensive coordinator, which he served as for Rodriguez for five seasons.
In his first season as defensive coordinator, Casteel's Mountaineers ranked 74th in total defense and were particularly bad against the pass, as they finished that season with the nation's 104th ranked pass defense. However, his defense did have the country's 34th ranked rushing defense and 44th best scoring defense.
With the exception of the rushing defense, which slipped to number 50, WVU improved immensely on the defensive side of the ball, moving up to the nation's 32nd best passing defense and 28th best team at keeping opponents off the score board.
2005 saw even more improvement, as the Mountaineers ranked in the top 19 in the country in rushing (19th) , scoring (13th), and total defense(15th).
The Mountaineer defense took a small step back in 2006, which had the nation's 13th best run defense but ranked 109th in passing defense. However, that trend was reversed quickly in 2007, the best season that Rodriguez and Casteel had while working together.
That year, Casteel's defense was the seventh best in the country and ranked number eight in scoring defense as well. The passing defense made a drastic improvement to number 14, while the rushing defense remained steady at number 18. Overall, the West Virginia defense ranked in the top 30 in the country in eight different categories.
When Rodriguez departed for Michigan, Casteel was set to join him in Ann Arbor before interim head coach Bill Stewart was retained. He decided to stay in Morgantown and remain the defensive coordinator for West Virginia.
His defenses have been solid since Rodriguez's departure, including his 2010 unit that ranked in the top three in the country in rushing, scoring, and total defense and finished the season with the nation's 11th best passing defense.
Entering the Orange Bowl against Clemson, Casteel's defense under first year head coach Dana Holgorson ranked 26th in the country in total defense, allowing its opponents 340.2 yards per game. Its strength is defending the pass, where the Mountaineers rank 30th in the country by limiting their opponents to 199.6 yards per game.
With his credentials as a defensive coordinator and familiarity with Rodrgiuez's system, Casteel is the perfect fit for UA's vacant position. He'll be charged with not only implementing a brand new defensive scheme, but also with turning around a unit that ranked 66th against the rush, 119th against the pass, 108th in scoring defense and 111th in total defense in the country.
While recruiting the class of 2012 has been a bit slow, Rich Rodriguez has been on fire in recruiting and hiring his assistants and has built a staff that will help him in his attempt to shift the power base in the Pac-12.