What was the structure built to keep out the northern barbarians called?

What would be considered a secondary source about the assasination of abraham lincoln

1. discovered the new world balboa 2. explored the north american southwest hudson 3. his crew was the first to sail around the world vespucci 4. conquered the incas of peru cortes 5. discovered the pacific ocean columbus 6. conquered the aztecs of mexico cabot 7. discovered fishing grounds off newfoundland pizarro 8. looted spanish vessels for england drake 9. claimed the hudson river area for dutch coronado 10. priest who explored the mississippi marquette 11. man for whom the new world is named magellan

Compare the lives of slaves in mines and farms versus those in households