Short answers, pulled from the story.
Most historians place the founding of Londinium shortly after the Claudian invasion of Britain, around 47-50 AD, with the earliest securely-dated structure being a timber drain of 47 AD. Some scholars defend an older view that a defensive enclosure was constructed during the invasion itself in 43 AD.
Londinium's population is estimated to have peaked around 100 AD at between 30,000 and 60,000 people. After around 150 AD the population may have fallen by as much as two thirds, partly due to the Antonine Plague which swept the empire from 165 to 180.
Boudica, queen of the Iceni, destroyed Londinium in 60 or 61 AD as part of her rebellion against Roman rule. The revolt was sparked when Roman authorities flogged her, assaulted her daughters, and seized the wealth of the Iceni nobility after the death of her husband, whose will Rome refused to honour.
The Romans built the London Wall sometime between 190 and 225 AD. The wall was originally about 5 km long, 6 m high, and 2.5 m thick, and it survived for approximately 1,600 years, broadly defining the perimeter of the City of London into modern times.
Londinium declined rapidly in the early 5th century after barbarian invasions severed communication between Rome and Britain around 407-409 AD. By the end of the 5th century the city was largely an uninhabited ruin. The Saxon settlement that followed was established to the west of the Roman walls, in the area now called Aldwych.
Roman-era stretches of the London Wall are visible near Tower Hill tube station, in a hotel courtyard at 8-10 Coopers Row, and in St Alphege Gardens off Wood Street. Parts of the amphitheatre are on display beneath the Guildhall Art Gallery, and a section of the river wall is visible inside the Tower of London. The London Mithraeum, which dates from around 240 AD, has been returned to its original location under the Bloomberg building.