Circa AD 100, when London had been a city for at least fifty years, the Roman inhabitants built what we call a basilica approximately 500 feet long, larger than St. Paul's Cathedral. In its day, the building was not for worship, but served as administration buildings, housing a law court, town hall, and community center. Along side it was an open square, called a forum, where the townsfolk could gather with market stalls and simply offering a place to meet others.
The basilica was discovered in 1848 when Leadenhall Market was demolished in 1880 and Half Moon Passage was excavated. The remains of a large arch were what remained, and remains to this day, and can be seen in the basement of the present day barbershop at the corner of Gracechurch Street and Leadenhall Market (between Cornhill and Lombard).
Here is an artist's rendition of what it might have looked like. It was destroyed around AD 300 by Rome as a punishment for London's support of the rogue emperor Carausius.