Castonzo became the first true freshman to start on the Boston College offensive line since Paul Zukauskas in 1998, Castonzo helped hold opponents to only 61 tackles-for-loss (fifth in the nation) and 22 quarterback sacks in 2007. By protecting quarterback Matt Ryan at right tackle, he helped pave the way for the Eagles' 5,924 yards in total offense. Castonzo subsequently received All-ACC Freshman honors by The Sporting News and Freshman All-American honors by the Football Writer’s Association of America.

Prior to his sophomore season, Castonzo made the move from right tackle to left tackle where he started all 14 games in 2008. Boston College ranked 25th in the country in tackles for a loss allowed (4.77) and 49th in sacks allowed (1.69). Castonzo earned a College Football News All-Sophomore First Team selection.

In 2009, Castonzo was listed at No. 9 on Rivals.com’s preseason offensive tackle power ranking. Following a productive junior season, Castonzo was selected 2009 First Team All-Atlantic Coast Conference as voted on by the 40 members of the Atlantic Coast Sports Media Association.

Castonzo is a high effort player who consistently shows good overall technique. Has sound fundamentals, was well coached at BC, and uses technique to gain leverage on opponents in the run game. The 6 foot 7, 300 pounder works to get inside hand placement and plays with consistent knee bend, playing balanced and disciplined. Castonzo has ideal height and arm length for the position and extend arms at the snap and jolts opponents with his heavy hands. He is extremely effective in putting himself in proper position to handle the outside speed rush and shows enough quickness to consistently get to the second level and neutralize linebackers and safeties. Has enough foot quickness to re-direct on double moves in pass protection and plays with a warrior-like mentality.