The site for the new town of Londinium was probably chosen by the army. At first, the soldiers would have lived in a tented camp so that they could defend the crossing that was needed across the River Thames. Then after the Boudican destruction of the town in AD60, the army built a temporary fort in wood in order to organise the rebuilding of the town. This was replaced by a more permanent stone fort.

Although the fort was built in a similar layout to other known forts with barrack blocks for soldiers, stables for horses, granaries for storing food and headquarters buildings, it was built to house those soldiers who were in London not so much to defend the town but to help with the running of the province and also for those who were on their way to other Roman forts in the province. It would have held between 500 and 1200 men.

The army was also later probably responsible for building the town's defences. In about AD200, a huge city wall was built around the town, stretching for 3.2km from today's Blackfriars in the west to the Tower of London in the east. The ends were later joined by a wall that ran along the riverfront. The city wall remained standing after the Romans left and sections of wall still survive in the City of London today.