London began as a Roman settlement around 50 AD. The Roman London was set in the north bank of the river. But soon "Londinum" -as Romans used to call it- flourished and became the capital city of the most northern province of the Roman Empire. The Roman built a wall around the city to protect it from invasions and they also built public houses such as baths, temples, gardens, a church. In spite of that there are just a few remains of Romans in the actual London.
This photo shows a piece of the wall that the Romans built in Londinum and that remains at present.
This is the statue of a Roman general.
However, London went into a gradual decline caused by the fall of the Roman Empire and by mid 5th century they began to leave the city.