At the top of society is the king (born into his heritage).
Below him are the lords. The king grants them land (which there is precious little of) and in return they have to provide so many warriors to fight for so many days a year (for the protection of the king against subjects that rebel—the Hansians—and to ensure they are ready if an attack like that in the crusades arises.)
Chosen warriors are also often the ones sent out into the world to collect precious minerals they need to sustain their kingdom and make keys. They were chosen because they went through lustration and swore an oath of loyalty to the king. They became his vassals—they got accommodation and an allowance in return for allegiance.
Kings allowed the lords to pay 'Fire Money'. Paying warriors for their dragon fire when they needed them.
At the bottom of society are peasants. They cannot leave their kingdom without their lord's permission.
Furthermore, as well as working on their farms, they have to do the lord's wishes for 2 or 3 days a week.
Peasants have to work extra days for him at busy times like Harvest Fairs or pay rents. Peasants also have other burdens. For instance, when a father died his son had to give the lord the best piece of trade before taking over father's land.
The king rules by heritage. The Key that marked him by birth meant he would be a royal. Rebellion against him was a sin—was against nature. However that did not stop rebellions! Kings had limited keys in the Early Telluric Age and rebellion was easy. A great deal depended on the personality of the king. If he was strong he could control the barons/lords. If he were weak or indecisive the lords would often rebel. Warrior kings who fought successful wars were the most powerful as they were popular with the nobility.
Because the royal bloodline is important for seasonal traditions and conducting, when a warrior usurped a kingdom, they would keep royal line alive but held captive, using their blood in the required weekly rituals.