My apartment is on the edge of what they call “The City”, which is short for The City of London. Nowadays it’s the financial center of London, full of shiny modern skyscrapers (which may or may not look like pickles and/or walkie talkies), but back in the day this was it — THE city of London that goes all the way back to the fortified town the Romans built in 200 A.D.
There’s an official London Wall Walk that essentially consists of series of panels that mark where the old city walls used to stand (or even in some cases still do stand). They start at the Tower of London (built in the 1060s after the Norman Conquest) and curve up and around to a complex called the Barbican, which stands on the site of the Roman fort.
Finally you end up at the Museum of London. The drab and somewhat grimy-looking building vastly undersold the quality of the exhibits — for a free museum on a hot Saturday, I thought it was pretty good. The museum walks you through the history of the city, from the Romans to the Middle Ages to the Black Death and the Great Fire. It weirdly skips WWII (I assume they’ve got other, better museums for that) and wraps up in the 2000s with the Olympic Cauldron.
If you haven’t seen this yet, CGP Grey’s explanation of the City of London is most informative! https://www.youtube.com/watch?v=LrObZ_HZZUc