London is marked by its history.
For almost all of it the key factor was the bridge(s) across the Thames. First built by the Romans, it made Londinium a trade centre, and a sea port. The bridges across the Thames remain major features. As London grew it absorbed previously separate villages, and even another city, Westminster. Some of these have simply vanished, others retain something of their character, or at least a name. Layer upon layer of history make London... Once a walled city, "London wall" is still a street name and there are Roman and younger traces still standing.
Westminster Abbey was a marvel before the Normans came. They added the beginnings of the Tower of London.
Royalty left their mark, from palaces to hunting grounds and parks... The amount of green open space in central London remains exceptional. But few people were at the top of the social pyramid and at the dirty end of the river, and the downwind end of the city, were the slums: "The East End". It's still not as gentrified as the west, even in the days of cleaner air, and a sewage system that was the marvel of all, when it was built. Then there was fire: The great fire of London changed its face, as did the Luftwaffe in Word War Two.
Out of the ruins of each respectively came Wren's churches and 1950's civic development. More of the latter has since been demolished, but much of both can still be found. London, standing on soft clay not hard stone, was late to high rise buildings, but any modern description of London would include "The Gherkin" and Canary Wharf, as well as The Houses of Parliament and St Paul's cathedral.