The defensive action of walling new Roman townships may have possibly been taken by one Clodius Albinus in order to protect the towns in 196AD, when he withdrew his legionaries from policing certain areas. Later in the early 3rd century, the replacement of these earthworks with stone walls was gradually started and continued for the next fifty years, the great civil engineering project meant that by the 4th century all the towns had thick stone walls, backed by earth banks with defensive ditches in front. The dating of the walls of London is an interesting mystery, the evidence of coins dating from 183AD, together with pottery deposits, shows that the walls of London could not have been built before 190AD, and could have been constructed as late as 210AD.
Saxon raids began to threaten towns on the coast, Rome's response was to build the forts of the Saxon Shore, this process was very expensive, and the empire as a whole was going through a desperate period of economic and political upheaval which nearly destroyed it in the mid third century.