The city of Rome at its height had a population of somewhere between half a million and a million people — the estimates vary and the ancient census figures are difficult to interpret — compressed into an urban area that had no master plan, no grid, and no effective building code until the fires that made such codes politically possible. The vast majority of these people lived not in the marble houses of imperial imagination but in multi-story apartment buildings called insulae — islands — so called because they filled city blocks the way islands fill water, surrounded on all sides by streets. The insula was Rome’s residential reality, and it was often dangerous, frequently squalid, and occasionally catastrophically flammable.