Roman architecture brings temples, baths and amphitheatres to mind; yet the building type that housed a population approaching a million in the imperial capital was the *insula* — a multi-storey block with shops on the ground floor and rented flats above. Against some 44,000 insulae, Rome counted only about 1,800 *domus*, the single-family house. In Rome itself these lie beneath later building; in the port city of Ostia, whole quarters survive.

The typology is strikingly modern. Load-bearing brick-faced concrete walls, four or five storeys, wide windows onto the street, rooms facing an inner court or light well, and shops with their own entrances at ground level. Lower floors were dearer; since water and latrines reached no higher than the ground floor, the upper storeys were both cheap and dangerous. Augustus issued a regulation limiting height to about 21 metres against the risk of fire and collapse, and after the fire of 64 CE Nero tightened the rules and required space between blocks — among the earliest known building codes.

Ostia matters because what survives there is the ordinary. Bakeries, taverns (thermopolia), warehouses, guild buildings and flats of different income levels can be read side by side; most of what we know about the daily working of a Roman city comes from here rather than from Pompeii. From the third century the city lost its harbour function to Portus and was gradually abandoned, and being covered by silt is why it survives. Multi-storey rental housing, a shop level tied to the street and dense urban fabric were not built at this scale again until the European apartment block of the nineteenth century.