(Photo: Junfu Han, Detroit Free Press)
The junior forward's name was in the portal in the early hours of Thursday morning.
Castleton, who just wrapped up his sophomore season, was a backup big man who appeared in 25 games this season, averaging 7.9 minutes.
He averaged 3.1 points, 2.4 rebounds and 0.5 blocks while seeing time at power forward and center.
The former four-star prospect saw his role dwindle this season. After averaging 12.4 minutes through the first five games, he played single-digit minutes — or no minutes at all — in 15 of the ensuing 21 contests.