The city wall was built in about AD200 and encircled Roman London on its northern side. It stretched for 3.4km (nearly 3 miles) from modern-day Blackfriars in the west to the Tower of London in the east. After AD350, at least 22 semi-circular towers were added to the eastern section of the city wall to provide platforms for catapult machines (ballistae), the bases of several of which have survived. Enlarge the plan of London showing the passage of the wall - the red markers show sections that still survive.
The walls were built of ragstone, quarried in the Maidstone district of Kent, and brought up the Thames by boat. On the external face of the structure, at ground level, a chamfered plinth 2.7m thick was laid, made of huge blocks of brown sandstone.
On the internal face, on a level with the external plinth, three courses of flat clay tiles were laid, with an offset of almost 8cm between the top and middle tile. More than a million squared blocks of ragstone were laid in regular courses like bricks. On both faces of the wall, squared blocks of ragstone were laid in regular courses, while the core consisted of ragstone rubble, around which lime mortar was poured. Regular bonding-courses of two or three layers of tile were laid across to reinforce the wall. The original height is not certain, but it survived in recent times to a height of 4.4m (14.5 feet), and was probably at least 6m (20 feet) high when built.
Main gates set in the wall were Aldgate (the road to Colchester), Bishopsgate (Ermine Street to York), Newgate (the road to Silchester) and Ludgate (the road to the south-west). There were also two lesser gates from the pre-existing fort, one later known as Cripplegate and the other, the west gate of the fort – they were not main thoroughfares. In the late-Roman period Aldersgate was added, perhaps to replace the west gate of the fort.
The names of the gates are medieval and the Roman gates had successors that continued in use until they were demolished in the 17th and 18th centuries for road widening schemes. The six main medieval gates were Aldgate, Bishopsgate, Cripplegate, Aldersgate, Newgate and Ludgate. But William FitzStephen writing in the 1170s said there were seven and he specified these were along the landward wall. It is not certain what he meant by this.
There were also smaller postern gates, for pedestrians, like the one by Tower Hill. However, these are not usually included in any count of the number of gates. One of these posterns was at Moorgate which was pulled down in 1415 and a new larger gate built, so that in most counts after that, Moorgate became the seventh gate.
For further information, see about Londinium's defensive wall in Military life.