Hierarchical social structures mirror the medieval conception of the universe (or vice versa):

Great Chain of Being, an order created by God, not to be tampered with by man: god at the top and everyone else in descending order. People were expected to be content with their placement. If you were a serf it was because God wanted you to be a serf. Politically and religiously, this concept made people relatively easy to control.

God

Pope

Kings

Duke

Marquess

Earl

Viscount

Baron

Baronet

Knight

Serfs

animals

birds

amphibians

insects

 

Hierarchical structures are found in medieval literature, as well:

Click here to see a diagram of Dante's Inferno