Who built Persepolis, and were they slaves or paid workers?

Many were paid workers. The Elamite tablets found at Persepolis record silver and food handed out to the workmen of the royal economy, most often Cappadocians, Lydians, Carians, Thracians, Ionians, Sogdians, Bactrians, Babylonians and Egyptians. Read from those records, the workers who built Persepolis were paid for their labour, though paid is not quite the same as free.

Persepolis was the ceremonial capital of the Achaemenid Empire, in Fars province of Iran. The earliest remains date from 515 BC, and it was Darius I who created it as the seat of government and the centre for receptions and festivals. His successors Xerxes I and Artaxerxes I extended it. Its exact function is unclear: it was not one of the largest cities, but appears to have been a grand ceremonial complex occupied only seasonally.

The proof is in clay. In March 1933, archaeologists led by Ernst Herzfeld found tens of thousands of unbaked clay tablets, the fortification tablets. They record food being received, stored and redistributed to travellers, workers, artisans and even livestock. Among Persepolis workers, there are as many women as men recorded in the archive. Some women got more rations than any man in their group, due to their rank or special skills. New mothers got rations too, and mothers of boys received twice as much as mothers of girls.

A later set of tablets, the Treasury Archive, records silver payments made in place of rations of sheep, wine or grain to workers and artisans.

There is a caveat. The fortification archive records only food transactions, so it limits what we can deduce, and it does not settle every question about who was free and who was not. But the evidence we do have points to an organised workforce that was paid in food and silver.