The history books have a very precise date for the end of Roman Britain. In 410AD the superpower refused a request from the British for military aid and Britain ceased to be a Roman province. Within a generation Londinium – today the vast area that is the City of London – was abandoned and most of its inhabitants had retreated to the countryside.

For some five centuries following the collapse of Roman Britain there is little evidence of life in the area of the former Roman provincial capital until King Harold re-occupied it in 886AD in response to the threat of Viking attacks (Bede referred to the abandoned Roman city in his writings in the 8th century AD). The walls that had enclosed Londinium then provided people with the protection required from Nordic invaders.

Mocked-up scene of tumbled blocks at the Museum of London, depicting the end of Londinium.

In reality, Londinium didn’t collapse overnight, but faced long decline, which some have suggested could have started as early as 140AD, only a century after it was founded.

“For its first hundred years Londinium was a town of spacious public buildings but cramped, cheaply constructed houses and shops,” Francis Grew of the Museum of London has written. “For the next two hundred it was one of decaying public buildings but large, well-appointed private residences.”

To discover more about the decline and fall of Roman London, I went on a walk guided by former Museum of London archaeologist Kevin Flude. He compared the collapse of Londinium to Detroit, where “property has got no value, so the city begins to shrink in particular areas.”

In the dying days of Roman Britain, residents couldn’t sell their houses, so the properties were simply abandoned and often neighbours used the vacated plots as rubbish dumps or to extend their own homes. Over time, this activity multiplied, leaving vast swathes of park land and rubbish dumps.

From a population of 35,000 at peak, it fell closer to 15,000 and then kept on getting smaller and smaller, leaving today huge volumes of dark earth.

But why did Londinium fall into decline?

To understand this, you need to look at why the settlement was founded in the first place.

As we walked around the western parts of the City – north of St Paul’s and close to a former Roman fort – Flude described how Londiunium was designed as “a strategic place for controlling and exploiting the province” and how it was “city of pen pushers and a city of bankers.” Interestingly, the most excavated item from Roman London is apparently a stylus.

Londinium was home to the main manufacturing and distribution centres, as well as also being propped up by government spending. But once the British economy became established itself, Rome dealt directly with towns across the province. Many goods were sent direct to York or Winchester for example, rather than being sent via London.

“Roman power was originally concentrated in Britain, but then it spreads out across Britain,” said Flude. “It just becomes an important city, rather than the be all and end all of the Romanisation of Britain.” And with government spending reduced in Londinium, the “economy can’t support a big city,” hence the de-population.

We walked around by parts of the old Roman city walls, which as I have noted before were only erected in about 200AD when Londinium was supposedly in decline. And it came at the time when a fortress – found after the Second World War near where the Museum of London is today – was abandoned.

London Wall – a modern road created at the northern most part of the old Roman city.

The main public bathhouses were demolished around 180AD and the basilicalla (traces of which can be seen in the basements of buildings around Leadenhall Market) was derelict by 300AD. In addition, the main amphitheatre is unlikely to have been used much beyond 350AD.

On our walk, Flude noted that the site became a “boggy hole”. He said: “At the end of the Roman period the amphitheatre was dismantled, not in one fell swoop but people start robbing the stones and the stones are used elsewhere.” It is also possible that Christians were martyred at the site, while others have said it became a cattle market.

Guildhall Yard

But for all the talk of Londinium in the history books, the settlement was only really boomed for a relatively short period of time. It has left little legacy and there is barely no connection between the Roman city and the one we now see today. Nevertheless, it is still a fascinating place to discover.