(Later Edit: Clarifying my question to could Rome, its empire, its complex urban civilization, grandeur, and vast public works be built without slavery in its era or the middle ages?)
Could a civilization at the scale of the Roman Empire in its period exist without slavery? I am looking for technology including medieval period technology that could have allowed it. I am trying to create a realistic Roman Empire style civilization without slavery in a world that mixes classical era and middle ages technology.
The Roman Empire's 'greatness' was essentially built on slavery and surplus wealth created by slavery from agriculture, mining, and the spoils of war. It was also built to a lesser extent on trade, uniformity, unified monetary policy as the primary minter of coinage, and monopolies on trade. There was the Pax Romana it created to create a wealthy complex civilization with widespread trade networks, but this was built in Rome which was built on slavery. The later Eastern Roman Empire aka Byzantine Empire would build its wealth on trade but be significantly less powerful as well as less wealthy than Rome.
Of course, technology played a part in Rome's glory, but the material wealth from slavery allowed a complex society to emerge with relative luxury in urban areas. This then allowed such technology and skilled craftsmanship to be developed in a complex society. The patrician class often owned large plantations and mining operations where slaves toiled away. Julius Caesar once sold 53,000 Gauls to slave dealers on the spot as historically recorded. Slaves were divided into several categories such as prison labor and prisoners of war.
Slaves were needed to build the massive public works of Rome, engage in large-scale mining operations, and engage in 'factory farming' with large-scale monoculture plantations.
The excess wealth of the patrician class necessarily built on slavery allowed taxation which expanded the public purse and later private purse of the emperors. This wealth was then used to pay the legions of the Empire and construct the massive public works, baths, monuments, palaces, villas, etc. Often the wealthy patricians and emperors would build the great works that are the hallmark of Rome with their funds. The patronage of the patrician class funded skilled craftsmen, artisans, architects, thinkers, and others in creating the advanced 'culture' Rome possessed. Such great urban civilization was not truly seen again for centuries.
Could there be such an Empire in that era absent slavery? One could say it could be serfdom instead of slavery, but serfdom is a somewhat different institution. What technology or social structure could make it possible? Trade monopoly alone would not make for a very realistic world, and the Eastern Roman Empire aka Byzantine Empire, as well as the greatest mercantile republics in the Middle Ages, did not possess the glory, wealth, or power of Rome. Nor were they able to create the 'Pax Romana' that allowed the Empire to have massive cities with complex trade networks existing in relative peace.