What I'm thinking off is just civilians, perhaps a conquered town, set to work on empire business (to be decided by me at a later point...) or work in a fort/walled administrative town, so hopefully that would be plausible.
A number of options, then. If subjugated, the conqueror might not wish to enslave them all. After all, the businesses in the town could still be profitable, os therefore eligible to be taxed to the empire.
However, those who were enslaved (be that a token amount, or the whole town (which might drive down the overall value of a slave on the market*)) could be utilised as common labourers (in some cases punishment for senior officials, making them do manual work), or in jobs where their talents gave the most efficient service to the empire or the local commander.
For instance scribes and tallymen in Ancient Rome were often slaves. You'd also have domestic servants, gardeners, farmers (there are still urban farms in places, but in the past keeping livestock was much more common and some towns had common grounds for grazing), and specialists such as bakers and blacksmiths. They could even be offered the chance to work their way out of slavery with good service, and if they reach 30, as per springs post.
It does sound plausible, by the way.
*Such comments of people as chattels make me shudder, even though pertinent.