Under the forecourt of the Guildhall
City of London

This amazing ampitheatre was only discovered as recently as 1988.  The site where the ampitheatre was discovered during building works became instantly famous and was designated a protected monument.  The Corporation of the City of London (London's central business district) was keen to integrate the remains into its proposals for a new art gallery, and in 1992 work started to excavate the whole site which took 6 years

London's Ampitheatre was part of Roman London which was built in AD47, on the site of two small hills (on which now sit St Paul's Cathedral and Leadenhall market).  The site was selected because it had a fresh water supply, wood and stone for building and easy access to the sea via the River Thames. 
The new city was called Londinium and it quickly became the largest most vibrant place in the whole of the Roman province of "Britannia".  In the next 100 years Londinium became famous for its wealth of traders and commercial traffic and at this time all the major public buildings like the Forum, Basilica, public baths and the Ampitheatre were put in place.
The Ampitheatre was used for the entertainment of the population of London, including soldiers, for performances for public holidays, professonal gladiators, animal fights and public execution of criminals - all of which constituted entertainment for the masses in Roman London!
When the Romans left London, the city developed over the next 2000 years with buildings being erected on top of the Roman originals and it was not until some major excavations were done that the extent of the Roman remains were discovered.  Well worth a visit, as is the Guildhall Art Gallery next to it.

 Nearest Tube Station - Bank - Central Line