London was (almost entirely[1]) burned to the ground in the great fire of 1666[2]. The new city was built on the smoldering wreckage of the old one.

[1] Pretty much the only things that survived were the tower of London and a few streets around The Cathedral of St Bartholomew the Great near Smithfield Market which most people even in London don't know about even though hundreds of people pass just one street away every day https://en.wikipedia.org/wiki/St_Bartholomew-the-Great

[2] https://www.london-fire.gov.uk/museum/history-and-stories/th...