The bombing that ravaged Victorian London brought Roman London to light. When, after the end of the Second World War, the debris of Victorian London were being probed in order to find solid ground below them to carry the foundations for ponderous new buildings, these soundings revealed almost the whole of the tracée of the Roman city-wall, of which, previously, only a few fragments, here and there, had been located, and a Roman temple, dedicated to the god Mithras, was uncovered  close to the starting-point of Watling Street, the Roman road that ran diagonally across Britain from Thames-side to Mersey-side. These excavations gave the measure of the rise in the level of the surface of the City of London within a span of eighteen centuries. It is impossible to estimate how much of this rise was due, before the Second World War, to deliberate destruction and how much to natural decay and to the excess of intake over discard which is a normal feature in the life of any city. The fate of London after the Roman evacuation and during the English invasion is unknown, and we also lack precise information about the extent of the destruction that was the price of London’s defeat, in A.D. 895, of a Danish armada’s attempt to force a passage, past London, up the Thames.
Wikipedia: “The first extensive archaeological review of the Roman city of London was done in the 17th century after the Great Fire of 1666.”
It was founded c AD 50 after the Claudian invasion. Ten years later it was sacked by the Iceni led by their queen Boudica. It was at its height in 122, when Hadrian paid a visit. The Wall was built between 190 and 225. By then, it was declining somewhat, perhaps as a result of the Antonine Plague. There were Romano-British as well as post-Augustinian bishops of London.
London passed from Middle Saxons (whence Middlesex) to the Kings of the East Saxons (Essex, regnabant 527-825) and/or their overlords, the Kings of Kent (regnabant fifth century-871) or East Anglia (regnabant sixth century-869) or Mercia (regnabant 527-918, but as client kings of Wessex from c 879).
The 895 armada was neither the first nor the last Danish attack. London was at the southern edge of the Danelaw. The Danes controlled it directly between 871 and 886 and later under Cnut. After the first occupation, London was reincorporated into Mercia. Mercia was then absorbed by Wessex (durabat 519-after 925).
Cities on the Move, OUP, 1970