The Roman city
at the site of the present-day city of London
, established in 43 AD
The original town was destroyed by Boudica's uprising just seventeen years after its founding, but rebuilt on both banks of the Thames after the revolt was defeated. By 200 AD, Londinium was capital of "Britannia Superior", with walls nearly seven meters high. (York ruled over "Britannia Inferior"... and ask any English person if those names don't resonate to this day!)
Through invasions and imperial secessions Londinium retained its Roman character...but by 400 AD the Romans had renounced their claim to the British Isles and withdrawn their legions altogether, under the onslaught of Celts and a few surviving Picts. Soon enough came a great migration of Saxons. By the seventh century AD the area was being called Lundenwic which rather brings the scope of this writeup to a close. Heh.
Sources: Museum of London, Encyclopaedia Britannica