Answer:
1) Fall of Rome
2) Lords
3) Serfs
Step-by-step explanation:
After the fall of Rome, all of the territories that were occupied and protected by the Roman Empire and their army were left unprotected and ready to be sacked by the barbarian tribes, this caused that people looked for protection form powerfull lords that would gather an army or a force to protect the people that would in exchange pledge loyalty to that lord and to its house, and would give a percentage of the harvest. Serfs eventhough were the ones that created and fed the entire society were the ones at the bottom of the social structure since they couldn´t defend themselves from invasors.