That pushes Masonry and Metalworking back awfully far though. I was thinking of switching the prereq for Slavery to Agriculture, with everything else staying as it is. Spoiler Wikipedia : Evidence of slavery predates written records, and has existed in many cultures. Slavery is rare among hunter–gatherer populations, as slavery is a system of social stratification. Mass slavery also requires economic surpluses and a high population density to be viable. Due to these factors, the practice of slavery would have only proliferated after the invention of agriculture during the Neolithic Revolution about 11,000 years ago. Good point on pottery and trade. I made it a prereq for Metalworking (which I'm renaming Smelting) because early copper smelting was probably done in kilns.