Deep in the bowels of the Guildhall Art Gallery are the remains of London’s Roman amphitheatre. Half a kilometre away, in front of the Museum of London on Noble street, are the fairly-extensive remains of the Roman city wall. Both are well-annotated and in reasonable condition.
The Roman Amphitheatre was constructed in AD70, shortly after the Romans arrived in Britain. By the fourth century, it had fallen into disuse and it remained so until a small section of it was discovered in 1988 by workmen excavating land for the Guildhall Art Gallery. It was opened to the public in 2003, and it appears that little expense was spared in making its presentation as interesting and evocative as possible. Visitors enter the darkened amphitheatre space through the same entrance that the gladiators would have used. Life-size pictures of gladiators and spectators are complemented by crowd noises and flickering lights. It is fun, and there are also some good annotation boards to explain what the little bits of rubble and wood on display are (a couple of spectator terraces, and the underground drainage canals).
Down Gresham Street from the Guildhall, a much more extensive section of the Roman city wall (constructed in AD200) is intact and on display. A viewing platform runs along the length of the grassy ditch facing the wall, and it is possible to get a good feel both for its scale and for the skill behind its construction. There are several annotation boards along the way, and I learned that the wall stopped the northward spread of the Great Fire of London in 1666.
To visit the Amphitheatre you need to visit during the Guildhall Art Gallery opening hours (Monday to Saturday from 10am to 5pm, Sunday from noon to 4pm). Entry is GBP2.50 per person, and is free after 3:30pm and all day on Fridays. The Guildhall Art Gallery is occasionally closed for official functions. The Roman wall is open at all hours, and access to the viewing platform is unrestricted.