In the recent game Assassin's Creed: Valhalla, one of the locations is the city of Lunden circa the late 9th century CE, about 400 years after the Romans left. The city does look distinctively Roman, with a standing amphitheater, a villa which is still used, statuary, fortified walls, frescoes, and plenty of ruins throughout. The landscape in the rest of the game is also dotted with plenty of Roman buildings and architecture in various states of repair. I assume the creators took some creative liberties when designing the environment, but historically in other games in the franchise they have done a lot in the ways of presenting places accurately. London today obviously doesn't still have these structures, and does not have much standing Roman history remaining to my knowledge outside of some walls. How long were these large structures used by the successors of the Romans before being replaced?