What was the Roman insula?

Roman insulae were apartment buildings that formed a major part of the housing stock in large cities, especially Rome itself. They often had shops on the ground floor and apartments above, with wealthier residents occupying lower or better-built units. Some insulae rose several stories high, making them important for fitting a large population into limited urban space. Because of rushed construction and fire risk, many were vulnerable to collapse or burning. They show that ancient Rome was a densely populated and highly vertical city.