So, originally Ostia was a naval base. Soon, however, many craftsmen would settle down in Ostia for making a living in and around the harbour.
After the wars, life in Rome became more stable. The military nature of the colony diminished too. The population of Rome increases and so the necessaties of life. Ships come and go to solve these needs. Many of the vessels had to tranship their cargo at Ostia. Logically an active trade appeared in the harbour town. Traders started their trade offices, shops were opened and the local middle-class increased explosively. At the height of its prosperity, in the second century AD, about 40,000 people were housed in Ostia. The city has developed then as a real city with its own temples, bathhouses, a theatre, shops, warehouses, construction places, workshops, guilds etc.