From coin evidence, the wall was built between about AD190 and 220, the evidence coming from a forger, who was busily making imitation silver coins (denarii) in London in about AD200 and which he threw away, together with a genuine new denarius of 213-17, in rubbish that had accumulated under the stairway of a tower in the city wall in Old Bailey.
The wall was built using squared blocks of ragstone laid in regular courses, with a core filled with ragstone rubble and lime mortar and a flint and clay foundation. At regular intervals, bonding-courses of 2 or 3 layers of red tile were laid through the thickness of the wall as reinforcement. Taking the course of the Walbrook stream into account, brick-lined culverts were made through the wall, where necessary, to take natural drainage.
The original height is not known, but it was probably at least 6m high when built. It survived in recent times to a height of 4.4m and one portion was recorded as having stood nearly 5m high. The line of the city wall was laid so that it joined the north-east and south-west corners of the fort, and the north and west walls of the fort became in effect part of the new wall. The fort walls were, 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. From the time of its construction to the end of the Roman period the city wall itself seems to have undergone little change.
In front of the wall, and separated from it by a level space, 3 to 5m wide, was the defensive ditch. This was wide and V-shaped in section, and was about 5m wide and about 2m deep. The earth dug from this and from the wall’s foundation trench was eventually piled up against the inner face of the wall to form a great bank, which added to its strength and prevented the wall from being pushed inwards.
When towers were added to the wall in the later 4th century (see Defensive towers), the old ditch was filled in and a new wider ditch, usually flat-bottomed, was dug further away from the wall.
A series of breaks in the city wall where the main roads led out of the town would have had gates. Aldgate and Newgate, may have been a pre-existing gateways that stood on their own spanning the roadway and were incorporated into the later city wall. When the city wall was constructed, there were gateways at Aldgate, Bishopsgate, Newgate and Ludgate.
Other gates, those in the north and west walls of the fort, would also have been in use but may not have been used as public thoroughfares. A later Roman gate at Aldersgate (between the site of the fort and Newgate) was added in the later years of the 4th century and may have been used instead of the gate in the west wall of the fort. However, actual remains have been too fragmentary to indicate the ground plans of the original gates.
For details of what can be seen today, go to City wall and gates in Londinium Today.