Wikipedia
An ergastulum (plural: ergastula) was a Roman building used to hold in chains dangerous slaves, or to punish other slaves. The ergastulum was usually subsurface, built as a deep, roofed pit - large enough to allow the slaves to work within it and containing narrow spaces in which they slept. They were common structures on all slave-using farms ( latifundium). The etymology is disputed between two possible Greek roots 'ergasterios' (workshop) and 'ergastylos' (pillar to which slaves were tethered).
The ergastulum was made illegal during the reign of Hadrian.
The term is also used to describe any small Roman prison.
Usage examples of "ergastulum".
He had hopes, too, of ultimately catching the good attorney napping, and leading him too, bound and docile, into his ergastulum, although he was himself just now in jeopardy from that quarter.
The general appearance is that of an ergastulum like Umm Amil: here perhaps the ore was crushed and smelted, when not rich enough to be sent down the Wady for water-working at the place where the inland fort now is.