There is evidence of several boundary fences and ditches which were constructed to protect early Londinium, before the building of a permanent stone wall to act as a formidable defensive structure that would continue to define the cities boundary for the next 1,500 years.
In the period of 180-220 AD a permanent large stone defensive wall was constructed surrounding the entire settlement of Londinium at the time, with the north and west outer-facing walls of the existing fort being incorporated into the line of the wall.
For a more detailed map of the Roman wall and its main features see my my Google Map.
On completion, the city wall was approximately 5km (3 miles) in length, up to 2.4m (8ft) thick and up to 6-7m (20ft) tall and enclosed an area of 130 hectares (330 acres), making it the largest enclosed town of Roman Britain.  There would also have been a defensive ditch on the outer side and a raised embankment on the inner side.
The material used to build the wall was rag-stone, sourced from near Maidstone in Kent, and would have been transported up the Thames by barge. It is estimated that some 1,300 barge trips would have been required to transport the 85,000 tons of stone used for the construction. One such barge, fully laden with stone was found in 1962, sunken into the mud, at the entrance to the River Fleet, near Blackfriars.
The Roman method of construction for the wall used regular shaped rag-stone blocks, for the outer faces, with rag-stone rubble infilling the inside core of the wall. At every 3-5 courses of regular blocks, a “bonding layer” of red tiles was laid across the whole width of the wall to provide additional strength, and to act as a moisture barrier, like a modern-day damp-proof course. In the lowest foundation layers of the wall the bonding layers are more frequent and constructed of triple layers of tiles. At higher levels only two layers of tiles are used.
This construction method can clearly be observed at the original Roman lower layers of the wall where remains survive such as at Tower Hill and Cooper’s Row. Upper sections of the wall above modern ground level are usually of later Medieval construction, identifiable by the more hap-hazard arrangement of facing stones, and lack of bonding courses.
The lower third or so being of original Roman construction
During the 4th Century AD, the wall was enhanced with the addition of at least 20 towers or Bastions evenly spaced along the eastern section of the wall.  Each bastion was semi-circular and about 8-9m high and were built as an additional defence to provide a platform for an artillery machine called “ballistae”, essentially a large mechanical crossbow.
The outer skin of the towers was built from ragstone, like the walls, and the solid core of the tower bases included rubble and reused stone material including tombstones taken from earlier Roman cemeteries that were positioned on the outsides of the city wall at that time.
During the same period an additional river-side wall was constructed, running from Blackfriars to Tower Hill, along the line of modern day Upper and Lower Thames Streets. A section of the river-side wall is visible today in the grounds of the Tower of London.
Further reading (off-site)
- Wreck of Roman barge found at Blackfriars in 1962 on pastscape.org.uk