Insulae were four story walkup apartments. Built of brick, probably unplastered and little ornamented, they were entered from exterior stairs that led up, over a ground floor of shops, to corridors off which opened single rooms that were numbered. Each room had its own window of mica or selenite, translucent enough to remind you morning had arrived. Some rooms had small balconies for taking the evening air (and, most likely, disposing of garbage and night soil). My first known example of affordable rental housing through the ages.