The 2015 option would run the Colts $7.438 million, but this salary is only guaranteed for injury, so they could focus on a long-term extension in the time being.
The Colts had major offensive line issues when they drafted Castonzo a few years ago, and while he struggled a bit during his rookie season, he has shown improvement in each of the past years.
Last season, Pro Football Focus had him rated as the No. 27 offensive tackle in the league. Castonzo continues to struggle as a pass-protector, but he has been an above-average option in run-blocking.