Symbolism, Imagery, Allegory
Don't worry if you missed this one, since it's not given a lot of screen time. In fact, it's only rates a quick one-line mention in one of Samson's (many) laments: "Ask for this great Deliverer now, and find him/ Eyeless in Gaza at the Mill with slaves" (40-1).
So, Samson's labor camp is a mill, what's the big deal? A lot, actually.
The mill, an incredibly important invention in the history of agriculture and industry, was often seen to be a symbol of the threat of technology and of dehumanizing modern machinery that was taking us away from the honest, simple labor God desires.
For this reason, the mill became particularly associated with Satan, who's described as Mill-like in Dante's famous Medieval poem The Inferno, a poem Milton absolutely knew. Many years after Milton, the English Romanticist poet William Blake actually wrote an epic poem about John Milton(!) and in the prefatory poem to this epic, he coins the phrase "Dark Satanic mills."
You're probably getting the idea, then, that mills in general did not have super happy associations, and that it's no accident that this is the kind of labor Samson is forced to be doing.