Riot
For all that the wall kept them free, Hades knew his workers were primarily kept complacent through fear and work. The fear kept them from challenging him overtly, while the work made them too exhausted to try. But he hadn't stayed in business this long only to become complacent himself.
Despite his efforts, sometimes the workers whispered, conspired against him. When whispers became shouts of defiance, of refusing orders given, that was when Hades had a riot on his hands.
The workers' plans of attack were disorganized and sloppy. Sometimes they would attack their fellows, trying to overcome his foremen by force, while other times they turned the very tools they worked with against his machines.
The most irritating times were when they simply refused to work at all, for even the carnage they sowed was at least doing something. But a well-placed order to the guards or a temporary concession to appease them, soon walked back, tended to end it.
And when Hades deigned to show up in person, by the time he strode through the doors to look the perpetrator in the eye, the riots were always quelled.