Castinus had the rank of comes domesticorum, commander of an elite unit in the Roman army under Constantius III.
However, the campaign was compromised at the very beginning when, according to one source, "his haughty and inept exercise of command" led to a quarrel between him and the military tribune Bonifacius, a protégé of Empress Galla Placidia.
Castinus continued on to Hispania, where at first he had considerable success against the Vandals in Baetica, managing to put them under a blockade and coming close to forcing them to surrender.
[4] If such an agreement was made, Castinus broke it when he joined in declaring Joannes, the senior civil servant, as the new Western Emperor in late 423.
The Emperor Theodosius invested his young cousin Valentinian III with the honor of Caesar the next year, then dispatched an army against Joannes.