London, as a permanent settlement, owes its origins to the Roman invaders, who in AD43 established the first buildings on the site; there is no evidence to suggest pre-Roman occupation of this marshy location. Since that date, London has enjoyed continuous occupation allowing archaeologists very little scope for large-scale excavation. In addition, occupation layers have resulted in a steady rise in ground level with Roman remains now buried some 10-20ft beneath street level, often obliterated by medieval and Victorian cellars and pits.
The original Roman London Bridge has been excavated, lying only 60 yards downstream from its present successor, and a series of major roads fanned-out from this crossing point as Londinium developed, becoming the principal trading centre for merchandise transported up the Thames and thence distributed throughout the province. During the early years of the invasion Colchester had become the principal Romano-British city and the Emperor Claudius intended it to be the capital of the new province. However, both towns came to grief in AD60/1, destroyed by Queen Boudicca and her Iceni armies, after which Londinium recovered the more quickly to usurp the pre-eminent position.
Neither fire nor flood (both were endured in the 2nd century) could prevent Roman London's rapid expansion. By the end of this century its walls enclosed 330 acres, making it the fourth largest Roman city north of the Alps and far-and-away the largest city in Britain, as it has remained since. In 197, Britain was divided into two provinces, Londinium becoming capital of Britannia Superior or Upper Britain. Urban life continued in the city well into the 5th century and possibly the 6th also, unlike a number of other urban British centres; only the so-called Dark Ages interrupted London's predominance in finance and trade.
Because of continuous occupation and over-building, not a great deal is known of Roman London and even less is in evidence above ground. The 2nd century Temple of Mithras is an exception; discovered near Wallbrook it was controversially dismantled and reassembled in Temple Court. Mithraism rivalled Christianity in the early centuries AD, being especially popular among the soldiery. With the exception of some stretches of city-wall, the surviving fragments of Roman London are isolated relics. The area lying within the bounds of the Roman walls now largely defines "the City" of London, its modern financial centre. At its peak Roman London probably numbered between 40-50000 inhabitants. Much of what archaeologists have been able to rescue from ancient Londinium have found their way to the British museum and the Museum of London, where they are on permanent display.