Alternately:

As the Golden Age faded into history, the era of the Steel Queens began. Over time they became less capable of maintaining the balance between authority and liberty, and the old rival houses began to accumulate power.

As the central political authority weakened, the nobles did everything they could to amass wealth. This lead to an era of predatory economic activity which created a fragile shell of prosperity at the expense of long-term economic health. All of the local markets were glutted with steel, iron ore, and coal, and those who had traditionally farmed had been lured into these industries by inflated pay. The city was being fed by expensive imports that the average citizen could no longer afford.

Then the Rust Monster Plague began. While the barons fought the plague, the local economy collapsed. Desperate for cheap labor, the lords turned to golems. Steel miners and soldiers replaced living ones, but the only ones profiting were the rust monsters, and the noble houses collapsed, leaving the city leaderless and under the military rule of those wizards who could command the golems.

Squeekarank arrived, or revealed itself from hiding, and slew or drove off the wizards. It was a rust dragon of enormous power, and it lusted for the steel and iron that had been dug from the earth and used to build the city. It took command of the golem armies and used them to destroy the last of the population before dining upon them. It laid its eggs in piles of steel, ripped from the greatest buildings of the city and the larval dragons, (rust monsters,) devoured what steel could be found.

For a generation, Acheron was invaded by hundreds of wyrmling rust dragons as the satiated rust monster grubs estivated and metamorphosed into their adult form.

But in the ruined city the steel eventually rusted beyond usefulness for the rust monsters. Some still linger, half-starved and filled with hunger, gleaning splinters of steel from piles of rust. As for Squeekarank, none know her fate. Is she still there mining iron ore in the depths? Or has she left this world for the cube-worlds of Acheron?

Who can ever know? And who really cares?

(Seems to me to be a good place for a Steel Dragon Desciple to begin, and rust monsters would have no interest in the hoards of gold the nobles might have left behind.)