I've been listening to the podcast "the History of Ancient Greece" by Ryan Stitt, and in one of his episodes on King Darius I, he mentions that during the construction of his palace at Susa, Darius made a point of paying his laborers rather than using slave labor, as was usually custom. However, I can't seem to find a source on this. So, I have a somewhat multi-part question:
Did King Darius actually pay all of his laborers a wage during the construction of his palace?
And if he did, how common was this practice during his time? Was this a common practice among the Achaemenid rulers, or was this something unique to Darius?
Lastly, are there any other examples of large-scale construction projects that were built primarily by paid workers, and not slaves, in the ancient world?
Also, if anyone has any reading recommendations on the Achaemenid Empire, I would be very grateful for them!
So the short answer is "yes". The longer answer is that at least a large subset of them were. While I don't know if any administrative records relating to this construction (I find none in my source compendia), there are an abundance of records dealing with rewarding professional craftsmen (goldsmiths, "wood-handlers", masons, etc) at Persepolis estates, which the royal inscription at Susa indicates were present.
Dareios notes that the workers who did the back-breaking labour of digging and setting rubble for the foundation were Babylonian. This is interesting as corvee/conscripted labour is best attested in Babylonia. This is not exactly "slavery" but a form of forced labour. But without precise records it is hard to say with certainty. One problem with being sure is that while we have tons of records of workers of all kinds (at Persepolis) being compensated, there would of course be less likely to be records for workers not being compensated. However, I don't know of anyone who has argued that free labour was not the norm in the empire. There are records of slave sales and eunuchs (/u/Aithiopika provides some interesting examples here) but as far as I know, nothing suggesting the kind of armies of slave labourers we see in e.g. the late Roman republic.
There's also AFAIK nothing indicating Dareios was doing anything other than following the norms of his day. What the royal inscription at Susa does make a point of is that he brought workers and matrrials from literally every corner of the empire - Egypt, Khwarezm (approx. what we today know as Uzbekistan), Asia Minor, etc.
Apart from the better-attested construction works at Persepolis, I don't think wage labour construction work was all that uncommon in the ancient world, especially not by the 6th and 5th centuries. Even earlier, /u/Archsinner describes the Egyptian labour force who built the pyramids here. I don't have any great reference works on other periods myself, so I can't answer that aspect of the question in more detail.
For reading my main recommendation is Kuhrt's "A Corpus of Sources from the Achaemrnid Period". If you want a narrative history you could go with "From Cyrus to Alexander", but be cautious as Briant is not always all that careful in his usage of sources.