In about AD120 the fort was built to the north-west of the main settlement. It covered an area of nearly 5 hectares (12 acres). In plan it was almost square, over 200m along each side, with rounded corners. Its stone wall rose to a height of about 5m, and was reinforced on the inside with a thick earth bank. To the front was a V-shaped ditch, enough to expose intruders to fire from sentries positioned high above. On each side was a gatehouse with stone towers at each corner and at intervals between the corners and the gates. These rose above the sentry walk, providing additional vantage points for defending soldiers as well as space for stores or ammunition.
It was probably built to provide suitable barracks for all troops stationed in London and could have housed up to 1000 men. A century later, the fort was decommissioned and buildings dismantled. By that time the military situation in Britain had changed, and soldiers were now based elsewhere in London – especially on the south side of the Thames. The north and west walls of the fort were incorporated in a new town wall that was built around the whole of Londinium in about AD200. This ensured their survival for the next 1800 years. The line of the city wall was laid so that it joined the north-east and south-west corners of the fort. The fort wall was, however, less than half the thickness intended for the city wall, so the existing wall was heightened and a second wall was built as a thickening along the inside of the two fort walls, to bring this part of the city defences up to the standard height and strength.
For further information, see Londinium's fort in Military life.
See also Londinium's Roman Fort in relation to the remains of the fort and city wall today. This computer reconstruction was produced by Marco Bani and Kings Visualization Lab and was made as a post-graduate student project in 2008.