Per Wikipedia:

Despite being counted as one of the 28 cities of Sub-Roman Britain,[5][6] it seems that after the Romans left Britain in 410 Durovernum Cantiacorum was abandoned for around 100 years, except by a few farmers and gradually decayed.[11] Over the next 100 years, an Anglo-Saxon community formed within the city walls, as Jutish refugees arrived, possibly intermarrying with the locals.[12] The town’s new importance led to its revival, and trades developed in pottery, textiles, and leather. By 630, gold coins were being struck at the Canterbury mint.

Huh, I would have guessed “fire” instead of “abandoned for a century.” Neat.