Claim: London was founded in a year.

Londinium was a settlement established on the current site of the City of London around 43 . Its bridge over the River Thames turned the city into a road nexus and major port , serving as a major commercial centre in Roman Britain until its abandonment during the 5th century .   Following its foundation in the mid-1st century , early Londinium occupied the relatively small area of 1.4 sqkm , roughly equivalent to the size of present-day Hyde Park , with a fortified garrison on one of its hills . In the year 60 or 61 , the rebellion of the Iceni under Boudica forced the garrison to abandon the settlement , which was then razed . Following the Iceni 's defeat at the Battle of Watling Street , the city was rebuilt as a planned Roman town and recovered within about a decade . During the later decades of the 1st century , Londinium expanded rapidly , becoming Great Britain 's largest city . By the turn of the century , Londinium had grown to about 60,000 people , almost certainly replacing Camulodunum -LRB- Colchester -RRB- as the provincial capital and by the 2nd century , Londinium was at its height . Its forum and basilica were one of the largest structures north of the Alps , when the Emperor Hadrian visited Londinium in 122 . Excavations have discovered evidence of a major fire that destroyed most of the city shortly thereafter , but the city was again rebuilt . By the second half of the 2nd century , Londinium appears to have shrunk in both size and population .   Although Londinium remained important for the rest of the Roman period , no further expansion resulted . Londinium supported a smaller but stable settlement population as archaeologists have found that much of the city after this date was covered in dark earth - the by-product of urban household waste , manure , ceramic tile , and non-farm debris of settlement occupation , which accumulated relatively undisturbed for centuries . Sometime between 190 and 225 , the Romans built a defensive wall around the landward side of the city . Along with Hadrian 's Wall and the road network , this wall was one of the largest construction projects carried out in Roman Britain . The London Wall survived for another 1,600 years and broadly defined the perimeter of the old City of London .