Definition: Insula (pl. insulae) were frequently cheaply made apartment houses or tenements up to 6 or 7 stories high. They were fire traps, with no running water. After the great fire under Nero, the emperor restricted their height. Even if the insulae didn't burn, they might collapse from bad construction. Light came from openings we refer to as windows, although they would not have contained panes of glass.
Examples:In Colleen McCullough's historical fiction The First Man in Rome series, the youthful Julius Caesar of The Grass Crown lives in a memorable insula.