Ostia is generally believed to date from the second half of the 4th century BC, and was originally built as a military post to control and defend the mouth of the River Tiber. It takes its name from the Latin ostium, meaning river-mouth. In its heyday, Ostia was the principal port for the city of Rome and also a thriving commercial centre in its own right, with a population of around 100,000 people. Its decline began in the 2nd century AD, when much of the commercial traffic was redirected to the newly-built harbour at nearby Portus. By the 4th century AD the harbour at Ostia was beginning to silt up, and an epidemic of malaria eventually caused the town to be abandoned.
Ostia might be less spectacular than Pompeii or Herculaneum because it died a gradual rather than a sudden death, but it gives visitors a much more complete picture of life in a Roman town. Streets, forum, capitol, theatre, bathhouses (many still with their original stunning mosaics), temples, market, shops, offices, workshops, warehouses, grain stores and private residences - they are all here, and all remarkably well-preserved.
Ostia was home to all social classes. The wealthy enjoyed the sumptuous comforts of spacious detached houses (domūs), whilst the working-class people lived in the three- or four-storey apartment blocks (insulae) which varied considerably in their levels of comfort and decoration. One of the smarter ones is the House of Diana, which boats a private bath-house and a central courtyard.