Part of the reason the Achaemenid Persia is considered feudalistic is that we don't know much about the social structure besides that the king ruled through a series of other minor kings. There were three primary class divisions: warrior/aristocracy, priests, and farmers, though this structure was also influenced via descent from the father. The warrior class theoretically elected the king, who was the king of kings, as he exercised power through other tribal and local kings, hence the consideration of Persia as feudalistic. These other smaller kings controlled different regions through their own positions in society. Such a hierarchy was ever-changing with social and cultural influences from other regions, but the main "caste" structure generally remained the same, as did the king's exercise of power through what we would consider in medieval Europe to be powerful lords.