The game shows it in the 9th century as being fairly small (smaller than Jorvik), primarily Roman ruins, governed by a Danish ealdorman and with a large Danish population. If this is accurate, how did it come to be? I understand the coastal invasions of East Anglia and the more inland Danish conquests into Mercia, but how did London come to be that way if so? Thanks.
Hello, we recently discussed this very same question in our AC: Valhalla-themed AMA here.
The tl:dr of that answer is that the game is taking quite some creative liberties, as during the period in which the game is set, the Roman Londonium site (now the 'Square Mile' City of London) was abandoned for an original English settlement about a mile upstream (under modern Covent Garden) known as Lundenwic. Only in 886 did Alfred of Wessex restore the Roman fortifications and move the English settlement within them, entrusting the site to the Mercian kingdom ruled by his son-in-law Æthelred and daughter Æthelflæd. In 890, however, Alfred, Æthelred and Æthelflæd held a conference in London at which they substantially redrew the city's layout, and implemented a new street plan.
There was on occasions a Danish presence in Londinium. We know, for example, that in 883 Alfred led an army to protect Lundenwic from a Viking raiding fleet which had sailed up the Thames and overwintered within the Roman walls of the city. Indeed, this may have been the event which catalysed his restoration of the defensive circuit and translation of the settlement, as well as the establishment of a fortified bridge to the West Saxon garrison across the river at the South Work, modern Southwark.
In short, then, the game's depiction of London is a creative liberty. While Londinium did see Danish occupation, this was occasional overwintering by small forces within the relative safety of the surviving walls during a period in which the Roman city was abandoned in favour of the English wic, and not a true occupation of the city. For Londinium itself to be occupied and restored, yet still have a significant Roman flavour, the game would have to be in a very specific four year window between the restoration of the fortifications in 886 and the subsequent redevelopment of its urban layout in the early 890s. However, in this period, the city was very much in English hands with a large Mercian and West Saxon garrison (Southwark alone had a West Saxon garrison of some 1,800 men), and indeed was increasingly surrounded by an aggressively enforced English buffer.