With the departure of the Romans, the walled town was largely abandoned and the buildings would likely have collapsed, been consumed by nature, or pillaged for materials. The Saxons initially preferring a new location one mile (1.6km) upstream to be known as Lundenwic, where they settled in more traditional wooden structures.
Eventually, for defensive reasons, during the 9th century there was a move back inside of the remains of the Roman walled town, with Lundenwic becoming known as ealdwic “old settlement” whose name survives today as Aldwych, and is now associated to a street rather than a whole area. At this time the city walls were repaired and improved and the defensive ditches re-cut.
Even though the city walls were enhanced throughout medieval and even Tudor ages, the original line remained unaltered. The city wall was a prominent feature of the city for the best part of 1300-1400 years.
During the 17th century it’s defensive purpose effectively expired as the city grew beyond its confines, and the wall was gradually dismantled during the 18th and 19th centuries, as it was pillaged for building materials. Many parts of the wall becoming incorporated into the cellars and foundations of the shops and warehouses that grew in its place, or simply covered over and buried to make way for new or widened roads.
As for other Roman buildings within the city, much had collapsed or been dismantled, and what remained became covered over by natural means, or built over with newer structures. Events such the Fire of London in 1666 created mountains of rubble and debris. This was spread over the lower parts of London along what is now Upper and Lower Thames street, in order dissipate the material, with the effect of burying ancient structures under metres of debris.
During the late Victorian era, and into the early 20th century, many Roman structures were rediscovered as large portions of streets were dug up for the installation of sewers and railways lines. The slum clearances of this period also saw the demolition of large parts of the city to make way for wider roads and the building of banks, warehouses and offices.
This incredibly comprehensive publication of 1928, available to view online, gives great detail of all the finds from Roman London up to that date, with most coming from the period mentioned in the previous paragraph.
During the Second World War bombings of London, large parts of the city were destroyed. As these sites were cleared, sections of the lost city wall and other structures were rediscovered. The area of Noble Street and Cripplegate, has been left in this ruinous state, with the city wall revealed amongst the remains of destroyed Victorian buildings.