Roman insulae dominated ancient Rome's skyline as walk-up apartment buildings reaching up to eight stories. These crowded complexes featured ground-floor shops with cramped single-room units stacked above around light wells. Long before modern urban housing, they pioneered vertical living for a growing population seeking opportunity in the city.

Marcus Licinius Crassus exploited Rome's frequent fires, buying damaged buildings at low prices and rebuilding them with slave labor. The key innovation enabling these structures was Roman concrete, created by mixing lime with volcanic ash from Mount Vesuvius. This remarkably strong material could even set underwater, allowing construction on an unprecedented scale and enabling monumental works like the Colosseum.

Living conditions proved challenging. Fire hazards persisted despite height restrictions of sixty Roman feet imposed after the Great Fire of Rome in 64 AD. Tenants faced structural instability, poor sanitation with waste often thrown from windows, and the inconvenience of climbing hundreds of stairs. Unlike today's desirable penthouses, the top floors were the least desirable and cheapest housing options in Rome's vertical neighborhoods.