The governor of a castle.The castellan had recognized the seal of the Earl of Thierry on his letters of introduction....
French noblemen took to protecting themselves in fortified buildings that were known as castellans - these served as private fortifications in which people and animals were protected from these feared invaders
Until then, uprisings against the new Norman régime had been confined to local spats prompted mainly by the heavy-handed actions of overzealous castellans.
Origin
Late Middle English: from Old Northern French castelain, from medieval Latin castellanus, from Latin castellum(see castle).